You can find out if someone is lying by watching if there is a change in their body language, if they are unable to look you directly in your eyes when talking and also then check for general mismatches on a story. Find out more details here http://www.ehow.com/how_5145359_tell-person-lying.html.
if they look like they're going to laugh or smile, they dont want to look at you in the eye, if they can't stand still, if they flare out their nose, if they're voice volume changes by the words, if they play with their hair, if they put their teeth on their bottom lip, and/or they blink a lot. different people do different thing but these are the most common.
A few ways to tell if someone is lying are 1.If their eyes are darting around as you're interrogating them. 2. If they're fidgeting or using body language a lot more than usual. 3. If they get incredibly defensive when you say that you did it. (although this is not always the case. I get angry when people accuse of something I truly didn't due. 4. If the person puts something between you and their body i.e. Crossing their arms or holding a coffee cup in front of them.