Police and other law-related officials have a long history in detecting lies (especially on television and so on). How does it work? Is it as easy as they make it look? In short, no. But read on to see what I mean.
Firstly, the most important part of detecting deception is that you can’t. You can only suspect. To start, you need a baseline, i.e. an idea of how the person acts under normal circumstances. After that, you can then compare his or her behaviour when he/she is being asked a question to the baseline.
There are a few things that stand out as indicating deception.
When the expression doesn’t fit the words, deception is taking place, obviously. Therefore, to detect a lie, you have to make sure you know what every expression looks like exactly. For example, most people can’t fake a smile completely. If only the mouth smiles, and the eyes do not scrunch up (and make crow’s feet), the smile is not genuine.
Also, there is a whole department called micro expressions (popularised by Lie to me) that can also guide you. A micro expression is a very quick display of an actual emotion (people have no control over this) which can reveal another emotion as the one that follows it, indicating deception.
Choice of words
Apparently, people use words like “actually” and “never” in ways that can indicate that something’s up. When you use the word “actually”, you are comparing two things, so if the question raised no second option, there is likely deception going on. E.g. “Is your car blue?” could be legitimately answered with “Actually, it’s red.” But if the question is “What colour is your car?” and the answer is “Actually, it’s red,” it indicates that the answerer was comparing it to another colour, possibly the real colour.
The word “never” is often a way of avoiding the issue. “Did you steal the car?” could be answered with “I would never steal a car.” The answerer might be avoiding the answer and never really give a straight denial.
People who are lying often try to minimize the space they take up (by keeping their extremities close to themselves) and thus appear less dangerous—a survival instinct that kicks in for the same reason that a polygraph works (lying is directly connected with danger and the person is nervous).
When someone is lying, they tend to touch their facial features—the nose, ears, mouth and eyes. They also subconsciously place objects between themselves and the questioner or turn away from him, also an instinct to protect themselves.
The most important factor to consider when “detecting lies”, is that most of these signs could mean anything. Someone touching his nose could have hay fever and his nose is itching. Someone sitting scrunched might be cold. It’s important to take note of the context and work from a baseline, preferably in the same environment.
I got most of my information here.