there's a program on TV at the moment, about some kind of smarmy detective who can read body language. he 'learns the truth' from careful obeservation. if you do a google search on "body language couples micro gestures" (i was trying to find the research papers from a swiss group with an amazing success rate in predicting marriage outcomes) then you'll find tons of pages on how to read someone else.
i think this is all missing the point.
sure, you can tell a lot about what someone is thinking, and its great and all that, but the real power lies elsewhere (unintentional but apt pun).
learning about body language teaches you to learn your OWN truth.
so you find your eyes sliding to the left. what does that tell you about what you were thinking?
you are telling a story, and you rub your nose. why?
humans spend an awful lot of time lying to themselves. its that ego game, of constructing a desired personality and twisting the truth or downright lying in order to support the image. a trap we are all subject to, whilst bodybound, with its mental circuitry.
if you pay attention to your OWN body, then you can learn to hear your own truth. it can be distressing, so be gentle. the last thing you want is to start rejecting the information your body is supplying you. that is like telling a friend to get lost. it leads to disconnect which leads to illness and disease. for example. when i typed that last sentence i left out the bit about the friend. and i also accidentally left out letters in every word. noticing that i had omitted something important led me to rewrite.
So. pay attention. listen to your body, for it will not lie to you (unless you train it to as well. which would be a monumental fail!).
once you have mastered your own body language, other people become open books.