I don't know, I am still trying to figure that one out. I usually notice body language but then I usually tell myself I am paranoid or too suspicious of people. I wish I trusted myself more though; the worst feeling is when you don't go with your gut and then it turns out you weren't completely wrong.e.g. if someone's verbal communication and nonverbal communication don't agree, how do you come to this conclusion?
(This sounds funny but I wish more people would prove me wrong -- who wants to be right about crappy things?)
Have you ever heard of microexpressions? I've read psychology articles about how sometimes you might get a really strong feeling not to, for example, buy a car from some guy, and you might not know why you feel that way. But maybe he is trying to sell you a piece of crap and he had a smug expression for a second. A microexpression is a fleeting expression on another person that you aren't even consciously aware of. In the book Blink by Malcolm Gladwell, he talks about that kind of thing.
Maybe you already heard of that stuff... anyway, I just thought I'd throw that out there because I find it interesting.