This article does not so much tell you how to recognise that you are being followed or observed. That is a skill of instinct that one only acquires with time and experiences of being trailed. It is also a skill that must be learned differently in each country, because people who trail others are best identified by breaching subtle cultural norms, and each country has its own such norms. Nevertheless we can make some observations general to most cultures, and that will lead to a discussion of what to do when you are being followed.
The first thing to establish is why you are being followed. There are two principal reasons why a person might want to follow another one. One is to rob or otherwise cause harm to them; the other is because someone is paying them to do so (i.e. they are private investigator or a government intelligence official). These are two very different types of person. The latter is a professional; the former a mere criminal. People who follow you down the street to commit a crime against you are probably not very sophisticated. They will not have been trained how to avoid appearing suspicious. They will probably stop following you if you enter certain sorts of building: for example, bars, nightclubs or government buildings. They may stop following you if you proceed down a busy road. They are generally opportunists, more often than not people who see a foreigner they perceive to be rich.
These people are not as easy to spot as you might think; they can be more difficult to spot than professional private investigators or government intelligence agents. That is precisely because they have not learned their skills from a textbook; they have learned them on the street, and you have not learned your skills of detection on the street. This author will give one example of how a robber may follow you without your noticing. He was walking across a major African city, and not one of the more notoriously dangerous ones. He had decided to walk between locations (about 3 kilometres), because the traffic was so appalling. He was carrying a rucksack. He had been in a tourist bazaar, looking at trinkets. One of the people hanging round in the bazaar decided to follow him when he left. Given the population density, and the fact the author stood out like a sore thumb because he was the only white person walking through a black city, he did not notice at all that this person had been following him for 3 km. Another person then approached the author with a nonsense story of some kind; while that second person was relaying his story (the author did not stop walking) the person who had followed him was slicing open his rucksack with a Stanley knife. Of course this was a dangerous situation, because Stanley knives can kill people. The author had the good sense immediately to start running when he saw the other person; the only damage was to the rucksack. But the point of this story is that that the author, an experienced traveller in suboptimal environments, did not notice a 45-minute 3 kilometre trail. This is a long trail, and it took place in broad daylight. Professional trailers would not generally have the skills that this person had to enable him to undertake so extended a trail, knowing every backstreet, fitting in so intimately with the environment, and having the good fortune to be able to spot his target from a long distance off by reason of the colour of his skin.
By contrast, professional trailers are typically taught the private investigator's way. Depending on how many there are (a really sophisticated trail involves up to four people but they're quite expensive - say USD5,000 a day), they follow the routes you typically take and their aim is to collect information about you. They want to know what you do in the day, and where you do it. They may also be seeking to access your electronic information, by standing as a man-in-the-middle of a WiFi connection you might be using. Modern trailers therefore often have IT skills, as opposed to the ability to blend into a crowd. They are different sorts of people.
Also, remember that people who trail you may not have entirely malign instincts. Part of their mandate may be to ensure that you are safe and sound. In other words, they are bodyguards that you don't know about. This is particularly common amongst government intelligence services, who may be concerned about the physical welfare of a person but due to the clandestine nature of their operations they consider that they are not entitled to tell you that they are worried about you or that they are following you in order to keep you safe. These sorts of trailers usually have mixed motives, however. As well as ensuring your personal safety, they may also be asked to collect information on your habits, the places you go, the people you spend time with, and other personal information. People who are instructed to trail you in part to protect you may not be IT people, because IT people typically carry a laptop with them to try to break into your electronic communications; while people with a mandate to protect you probably won't be carrying a laptop because should violence occur, a laptop is an inconvenience. At the current time, hacking cannot easily be done in a sophisticated way from a mobile telephone. (However that may change with technological advancements.) Moreover the sorts of people used to violent situations and how to defuse them tend to be of very different builds and even personalities from IT experts.
Whether the professional trailer has as his or her primary goal physical protection, observation of behaviour, or IT intervention, their preferred location for their activities is a civilised cafe where they can sit down for long periods, waiting for the target (trailing people is extremely boring because you don't know when the person is going to come unless they are of the most singular habits and therefore you must wait around for a long time), and they must spend time setting up their laptop if that is part of their mandate. Alternatively they may be sending updated messages using their mobile telephones about the target's activities. In the interim they can drink a cup of coffee or a lemonade (professional trailers never drink alcohol on the job, naturally, as it impairs judgment) and sit in relative comfort. If a target is being trailed, and it is a serious affair, then over a period of days or weeks the habitual cafe-type environment in which the target uses will be identified and there will be somebody there during virtually all the hours that the target may be there.
Already we've learned a lot. If you see a person sitting in a cafe with out without a laptop and apparently absorbed in something, on their own (or two such people but not talking to each other and looking rather serious), and not drinking alcohol (or drinking very little in comparison to the environment), then you've probably found some of the people trailing you. If you know that environment extremely well, you can probably identify your trailers because each day you go there will be one of say four or so people there, rotated each day. Remember also (and this is a mistake often made by habitual experts in counter-intelligence) that they may be trailing somebody else - not you. If you want to check this hypothesis, move to a different cafe over a few days and see whether these people come with you!
We're now starting to learn how to identify the fact that we are being trailed, if indeed it is possible to do this accurately. To repeat, knowledge of local culture is imperative. In some countries, sitting on one's own in a pub is more or less unusual than in others; and people sitting on their own may have different sorts of looks and dress depending upon their purposes. The most difficult scenario in which to identify a trail is the "spur of the moment" trail illustrated by this author's walk through an African city. It's all very well saying "that person is following me"; they may just be going to the same place as you. Nevertheless if you think someone is following you, just change your location or take a taxi. Where you are dealing with a sophisticated government trail, you can't shake them off without separating yourself from you mobile telephone. That's because they're following you via triangulation of your mobile telephone coordinates with masts. This can sometimes, but not always, be defeated by turning your mobile telephone off. The only 100% effective methods are to separate yourself physically from your mobile telephone or to drain its battery to zero (the latter is extremely difficult with a lot of modern mobile telephones). One way to separate yourself from your mobile telephone is to go into a cafe and tape the telephone behind the U-bend in the toilet: the best place to store things in a public environment. You'd be surprised how infrequently toilet cleaners do their jobs properly.
For a more sophisticated private investigator or government intelligence trail, you will be trailed over several days or weeks, and you will see the same people rotating around you as the target in easy-going cafes and restaurants (i.e. ones where one does not have to spend a lot of money on food in order to sit there). The best course of action with a trail of this kind is simply to let it happen. If people want to follow you to a cafe, let it be. If your estranged spouse has sent a private investigator after you (and this is extremely common - so you should assume it so if you are reasonably wealthy and you have an estranged spouse), then they are looking for you in the amorous company of another person. Do not engage in such behaviour. They also may be looking for evidence of alcoholism. If your habit is to spend eight hours a day drinking 10 pints of beer in the same pub, change your routine a bit and consider cutting down on the alcohol; the trailer will be noting every drink you have, and possibly photographing you drinking it. But these sorts of thing aside, these trailers are harmless. What people do in cafes, restaurants and pubs is mostly harmless. After a while, if the purpose of the trail is not at least in part your protection, you will simply find that these people disappear because they have learned as much as they possibly can about you.
There are cases this author has known in which the people doing the trailing are apparently trainees, sent out to follow him because it is a good exercise. This author has some unusual venues that he likes to go to in an unusual city where he spends a lot of time, and it is sometimes surprising who shows up in such places. Should you be repeatedly trailed in unusual places, this may be what is going on. If you are always (or commonly) trailed at night, somebody may be concerned for your physical welfare.
It is fairly easy to shake off a trail - if you want to. Have a different mobile telephone that never comes home with you. (It is possible to connect two mobile telephones with no common accounts by tracing the movements of the telephones and finding that they are both associated with residence at the same address, so it is quite a lot of work to run two unconnected mobile telephones - they must also not share any connected email or similar such accounts.) Change venues and times of attendance constantly. This will vastly increase the amount of resources necessary to trace you. Some governments have limitless resources to do silly things like following you around; but private investigators do not.
If you want to have a meeting that nobody overhears, just arrange for it to take in a private building with telephones left at the door. (Sophisticated governments can turn on the microphone for a mobile telephone even if the telephone appears to be off, so if you really don't want government to listen to your conversations then you must be in a different room from the mobile telephone. One can of course have meetings in a giant Faraday Cage if you want 100% security - this author has worked for one public sector employer which used this and they are more common than you think.) So-called white noise generators no longer work, because the underlying speech can be parsed out from the white noise; technology has become ever more sophisticated. If you don't want anyone to know that you are meeting somebody, then you have to shake the trail. This involves not going out with your mobile telephone; and then taking the most arbitrary and capricious route to your destination that involves the maximum of crowds and the minimum of CCTV cameras, depending upon the determination of the person trailing you to follow you. Yes, it is a little bit like Jason Bourne.
But then always ask yourself this question: who are you? You probably are not Jason Bourne or anyone like him; pace a few Russian GRU assassins missioned to apply nerve agents to door handles, very few people are. Following people using CCTV is enormously complicated and generally only happens after the crime, unless the Police are tipped off in advance that a target is about to commit a serious crime. This, it goes without saying, you should not be doing. The point I am trying to make is that there is almost nothing even that professional spies do, that a trail can effectively prevent. Spies who meet one-another pass notes and information by pieces of paper that cannot be obtained by hacking a mobile telephone. The proper attitude towards trails for any person save an assassin or some similar heinous criminal (to which The Paladins are not in the habit of giving advice) is to tolerate them, however frustrating or annoying you may find them. It doesn't really matter; unless you are committing crimes or other indiscretions in public places, the person undertaking the trail is unlikely to obtain much interesting information that he or she could not obtain simply by asking one of your friends. (Your friends will talk if they are asked.) If the person undertaking the trail is in part concerned for the safety of the target, and you are the target, then be grateful. You might even buy them a drink. It's an awfully boring job and they are obviously in some way on your side.
Finally, some trails do get irritating; the person trailing pokes their noses into your life, trying to engage you in conversation or otherwise making your life unpleasant or different from how it would normally be. In those circumstances, complain. Complain to the person doing the trailing. It doesn't matter if you're wrong; the person will just think you're mad and that doesn't matter. "Please leave me alone; I don't want to see you or speak to you anymore" are magic words. If the person still doesn't get it, complain to somebody else - the manager of the venue where they are trailing you; their employer, if you can work out who it may be (again it doesn't matter if you're wrong; a complaint to the wrong organisation will just be discarded); somebody else who knows their employer and who will pass the message on; or ultimately the Police.
When dealing with a trail, stay cool. The job is boring and the trailer is unlikely to obtain any extraordinary information about you. You are who you are, as the old saying goes; and that is all that a trail is likely to find out about you. Although we all have a right to privacy, people have been invading one another's privacy since time immemorial and a trail is just another version of that. And remember: if you find it irritating, be patient. Eventually every trail just stops, because they realise there's nothing more to know about you. And in the meantime, imagine you're in a Jason Bourne movie and have a good laugh to yourself about it all.