Body language

Eyes definitely - if a girl keeps looking at you then something's up.

If a girl is going out of her way for casual contact (brushing arms, sitting relatively close next to you, etc), that is most likely a sign that she's interested. If it starts getting obvious, that's most likely her way of beating you over the head with a blunt object that says "make a move!"

And to add something to what dh2003 said about stuff like playing with hair - observe someone around their friends or people they're very comfortable with, if possible. That'll give you more information on what that gesture means - for instance, I play with my hair and half the time it's not conscious. All it means is that my hair's down and I'm not using my hand.

If your talking about women, look for these signs,
smiling is good thing
she touches you once she breaks the touch barrier your good
eye contact do not be afraid to look into her eyes makes you seem confident and admirable
when she flips her hair she likes you she's interested
playing with a jewelry she also likes you
Those are good signs
these are bad signs
crossed arms and crossed X doesn't equal the spot on this one trust me
no smiles bad news
rolling of the eyes also bad

How you hold yourself. Crossing your arms or legs could be sign that you are closed off. shoulders held low, no eye contact could show lack of confidence. Hand movement when talking is a good sign that you are interested in what is being said. Wetting of the lips can mean that one is nervous. Playing with hair could show that you are interested in the person you are talking to or that you are bored. Eyebrows pointing up means that a person is waiting for you to make a move. This could go on forever. hope it helps.