Difficulty: Moderately Challenging
Step1
The hand in front of mouth:
Does this mean someone is lying? No, not conclusively. If someone's hand goes instinctively to somewhere on their face before they speak this tends to mean that they are not sure whether to reveal something. This does not mean they are lying, although it is an indicator.
Step2
Saying too much or not enough:
These are signs that someone is trying to pull the wool over your eyes although usually an amateur. Saying too much indicates that they are nervous. Saying too little shows a level of fear not to give too much away.
While not conclusive I would watch this person for other signs of lying
Step3
Eyes:
These are usually the giveaway. An inexpert liar will probably not be able to look you directly in the eye while lying to you. However this is not how the eyes are the biggest giveaway with lying.
As a general rule when asked a question people's eyes flicker unconsciously either to the right or to the left. A look to the right is a look to the memory side of the brain, indicating they are recalling an incident that has occurred while a flicker to the left, the creative side, indicates that they are about to make something up.
This is pretty effective for deciphering what is being said to you using unconscious body language as it's very difficult for anyone to control.
Step4
There is one other way to know if someone is lying but you have to be quite expert at knowing what you're looking for. When someone lies, even if it's just a little fib, a small but perceptible smirk tends to flicker across their face for a millisecond that seems to say, I wonder if you're going to believe this.
If you don't believe me then try it for yourself. All you need to do is study the person's face in front of you. You may not perceive the smile but suddenly get a gut instinct that the person is lying.