They ussualy don't tak right, they don't look you straight in the eye, they move a lot and repeat things traying to find the right word. But then again there are those people who lie so much that they do it too well for people to notice.
Avoid eye contact, fiddle with something (hair, something in their hands, etc), their pupils dialate, they may give more than neccasary details that they usually would have left out, sweating (from nerves).....
They fabricate the truth, some people are nervous liars and often pass wind whilst telling lies so thats something to look out for
Here's a How to on how to detect a lie. http://www.funadvice.com/howto/detect_lie
lol i usally get p!ssed off when i lie, but i smile when i tell the truth XD
best thing to know it is by "watching lie to me " tv series
it depends what they have lied about ?