how to know if someone is lying?

Suppose that you went to a friend of yours with a fake smile on your face, and then told him that you were sad, would he believe you?

No, he would never believe you because he can see the smile on your face; in other words, he has noticed the inconsistency between what you are saying and your facial expression.

You might think "Nobody smiles when appearing to be sad” and you'd be right on that, but I’m not talking here about conscious inconsistencies; I’m talking about unconscious ones.

Your subconscious mind controls many of your involuntary functions like that of breathing, sweating, heart-beating....etc. when someone attempts to lie they usually control their facial expressions and tone of voice, but their unconscious mind still controls all other involuntary signals like body language and respiration rate. These unconscious signals are what we will be using to detect if they are lying; if their words are consistent with their unconscious signals then they are saying the truth otherwise, they are lying.

The following are signs you should look for when attempting to spot a liar:

• Body Language: Liars usually hide their palms and don’t sit straight while telling a lie. They also avoid eye contact at the time of the lie and generally use negative evaluation gestures if they feel bad while lying
• Eye Accessing Cues Mismatch: The direction the eye takes reflects the function taking place in the brain at the time. For example, looking towards the upper left side means that you are constructing an image in your mind while looking at the upper right side would mean that you are recalling an image. How can that be of use? Well, think about it, if someone looks towards the upper left when asked about something, he's probably constructing an image of the lie he's about to tell. This is just one sign that he might be lying.
• Inconsistencies in the Story: In different places and times, the story is more likely to change every single time it is discussed. He will forget a word, add something completely new, or remove something that he had previously mentioned. Regardless of the type of inconsistency, it shows he is lying.
• Feeling Anxious: Almost anyone who lies feels anxious, with various degrees depending on how professional the liar is. One of your concerns should be trying to spot his state of anxiety; talk as much as you can about the subject in order to keep him anxious.
• Avoiding the Subject: A person who wants to hide something will usually try to avoid bringing it up in the conversation or even talking about anything remotely related to it. Try to talk about anything related to the situation without addressing it directly and see if he tries to jump out of it into another topic.
• Lack of Assertiveness: Unless that person is a special agent who is professionally trained to lie, he will usually lack of assertiveness while lying. His tone of voice will be lower and he will appear to be less confidence
• Speaking Slowly: When lying, the person will usually be making up events as he goes along, which will result in slower speech or even several complete stops to think about what he's about to say.

It's very important to know that there are many other variables involved in this process. Thus, I suggest that you use these methods for decision support and not for sole judgment of others. For example, if you suspect that someone is lying, you should then start to see if any of these signs are present; of course the more signs you find, the more likely that person is lying.