It is not always easy to tell if someone is lying for the simple fact that some people are very good at it, for instance, actors (particularly method actors, some of the better ones e.g. Brando, De Niro, Pacino, can (could' in the case of Brando) even control or induce the physiological signs associated with, say, anger, grief, jealousy etc) and people who have been practising almost their entire lives and make a living from it e.g. politicians and lawyers (just a cheap joke promise I won't make any more after all, this isn't the humor section).
On the subject of 'trained liars,' we mustn't forget confidence tricksters who are essentially actors, sometimes very accomplished ones who can make a very good living from their talent. This is because with the right training, it is possible to control these physiological signs (mind over matter'), just as a yogi or fakir can assume voluntary control of vital signs that are involuntary in ordinary people e.g. heart rate, body temperature, pain threshold etc etc.
In fact, there are people who are so used to telling lies that they often don't even know themselves when they are telling a lie or who actually believe they are telling the truth even when they aren't. This is a characteristic of many young children which most people grow out of by the time they are adults, but in the case of some people, they never grow out of it and carry on the habit well on into adulthood most of these people go on to find very gainful employment in sales and advertising (sorry about breaking my promise, I couldn't help it). For example, I know an 8 year old boy who insists he has built an airplane himself and can fly it - as far as he's concerned, he is not lying, he is telling the truth. Children often have delusions of grandeur and far-fetched fantasies or will simply make things up to impress adults, win an argument or get one over on their peers: I bet my Dad could wup your Dad! he's 7 foot tall, weighs 30 stone and is a professional all-in-wrestler. So there!'
Even if such people or children - are connected to a lie detector or polygraph, it will not always show up because they believe what they're saying is the truth and so do not show the normal physiological signs of lying e.g. electro-dermal activity (sweaty palms), increased blood pressure, increased heart rate, perspiration, variations and constrictions in voice pitch and cadence, fidgeting, blushing etc. The theory is that practising deceit causes (at least in normal' people) a certain amount of stress in the body which must manifest itself in certain kinds of behaviour. Note: this does not apply to people who are pathological liars, for such people are invariably unaware of any conflict or contradiction.
My personal opinion is that polygraphs are probably 70 or 80% reliable but nevertheless open to occasional error. If someone is connected to a polygraph just for a job interview, it's possible that person may be so nervous in any case about being interviewed that they will show certain physiological signs often associated with lying. This applies still more to someone who is a suspect in a serious crime case and fears that they might be charged for a crime they didn't commit (which happens from time to time). For many years, I myself often had very sweaty palms that had nothing to do with lying, but more to do with a natural predisposition to sweat. I also used to suffer from eczema on my hands that caused them to sweat a lot, particularly when I was nervous for some reason or handling coins or chips which I did a lot when I worked as a croupier in a casino.
Tell-tale signs that someone is telling tales' are that they will tend to avoid making eye contact with the person they are speaking to, presumably for fear that the other person will be able to see into them' and tell that they are lying. Of course, another, more scientifically sound, theory based on brain neurology (and neuro linguistic programming) is that precisely the reverse is the case: a person who is lying will often look you straight in the eye without any eye movement up or down or to left or right because they don't have to access the true information in the corresponding area of their brain, but already have the false information ready to hand as it were (i.e. here's one I prepared earlier on'). This is perhaps counter-intuitive and goes against conventional wisdom.
On the more common sense level, a person who is lying will often look slightly embarrassed and uncomfortable at what they're saying or fiddle with their hands or fingers while they're talking, for instance making circles with their index fingers in some sugar lying on the table or rub their noses or ear lobes or scratch their heads or cough. These are indicators that most of us are taking in all the time at the unconscious level and that lead to a hunch or a gut feeling that someone is lying to us.
There's a lot of recent research that strongly suggests that when trying to decide whether or not someone is lying, the best method is to rely on or go along with our own gut feelings or hunches'. Our gut feelings have nothing to do with our guts but everything to do with the subconscious part of our mind that is far more efficient at picking up anomalies, inconsistencies and false notes in what people are saying and how they are saying it and other tell-tale signs, than is our conscious mind. There are a huge number of indicators, sense data and perceptions that we are taking in every second without our being fully aware of it or even aware at all.
So, to conclude, if we can only learn a few techniques along the lines that psychologists like Derren Brown recommend (eye movements') and at the same time train ourselves to have more faith in our own unconscious mind, we would almost certainly be able to make more reliable judgments about whether someone is lying to us or not.