This is one of my favourite subject, one can expertise in Body Language (BL) by observing people's behaviour again & again & again. Please note that the interpreting of Body Language varies from Country to Country, place to place, what is true in your place may not be true with other.
This topic is a vast topic, I will start addiding different types of BL in different headers. Let me begin with How to spot lies.
The classic eg is when a small kid, or a baby when it does something wrong, or it lies or steals, when asked by mother, the reaction would be it will close its mouth by its hands, & says it doesnt know about anything, this is hiding things, when the same kid grows up, he or she will not close their mouth, instead, it just takes its finger and touches its nose or rubs its nose, or it touches its mouth or forehead or just rubs its fingers over the face.
The classic eg I can give is Bill Clinton questioned in BBC news about Monica Lewinsky, just check the video in youtube or anywhere else, you can find his hand and fingers rubbing his nose continiously whenever he lied.
If it is an interview, try asking the candidate the same question in a different way, for eg, ask him if he is working, whats his salary, who is boss, what is his takehome etc etc, you can notice the body language as mentioned above if he is lying.
If you know the in & out of body language, then you can cheat the body language by not making any gestures while lying, it will be very difficult to notice for others.
