Definitely, you can tell a lot from watching people's body language, often far more than from what they say.
For example people look up in a different direction when they are telling the truth than when they are not! useful that!
Smiles as well, you can tell a fake/forced smile, often the eyes give it away.
I find it really useful when i'm communicating with deaf people or the hard of hearing, over and above sign language, body language can be far more instrumental in what you're trying to communicate.