Adding to Mr_Answers answer, some people also look away from the person they are talking to momentarily as they lie or continually as they continue to lie. People have a hard time looking someone in the eye while lying.
Also, shifting in their seat, changing the subject, looking down when accused...there are a lot if tells when people lie but be warned - people lie ALOT. When you learn the signs you may wish you can unlearn them!
There are various ways to tell if someone is lying. Some examples are, they hold there breath, their pupils get smaller or larger depending on the person, their palms get sweaty, or they momentarily freeze when they are telling the lie. Mr. Answer
Lie detection is very complicated, and you also have to have "base line" first; that is, you have to know what the person is like when he or she is not lying. One of the foremost researchers is Dr. Paul Eckman (the TV show Lie to Me was inspired by his work). His Web site has training on it to help people become more adept at detecting lies (similar to the "naturals" on Lie to Me). His Web site is www.paulekman.com.
It's a mistake to rely just on one or two supposed symptoms of lying, though. In fact, in criminal cases, that has led to some of "confessions" that were later found to be false.
You can tell if their lying when their overly nice giving such compliments as: I had a great time with you, sex was amazing let's do it again, and of course noooo honey your not fat and disgusting at all I think your so beautiful I would never cheat on you.