Lying makes people nervous and that nervousness conveys itself in many different ways. The better you know someone, the easier it will be to tell if they are lying to you or telling the truth. That doesn't mean that you can't often guess at how truthful even a stranger is being when you talk to them, though. Most people have some involuntary gestures that they make when they're trying to mislead you.
Being unable to make or maintain eye contact is such a universal signal of deception that it's entered into the popular parlance. A 'shifty-eyed' person is one who is not to be trusted. If you want to impress someone with your sincerity, look them openly in the eye. If someone keeps looking away from you - especially up and to the right - they may be lying or concealing something.
Touching the nose
Touching the nose is a nervous gesture that appears in children as young as two or three years old. People very often touch or rub the side of their nose when they are telling a mistruth.
Hiding or covering the mouth
Face touching is a very common 'untruth' signal, especially if the hands or fingers partially conceal the mouth or the lips. When someone is hiding something from you, they will often touch their chin, lips and cheeks. Be careful to put this in context though - in a courting or flirting situation, a woman touching her face and lips may mean something very different. It's important to gauge the entire interaction.
Rubbing the eyes or the side of the ear
Eye or ear rubbing is a common signal that someone is doubtful. You may see it when someone is trying to convince you of something of which they aren't entirely certain themselves.