BODY LANGUAGE

Body language, also known as kinesics, is a type of non-verbal communication in which physical behavior. Such behavior includes facial expressions, body posture, gestures, eye movement, touch and the use of space.

Here you can find some examples:

You can understand if someone is lying by these signs: to blink faster, to swallow more and to avoid eye contact. 
Eye contact is an indication of confidence, interest and honesty. Eye contact is also an important element in flirting, where it may serve to establish and gauge the other's interest in some situations.
Instead running your hands through your hair, especially taking hair away from your face, is often a sign of flirting and openness.  

Playing with hair is not always a good sign, though. Holding hair indicates awkwardness and nervousness but some girls twirl when they are tired, others when they are reading or watching television, and some when they feel nervous.