One common thought is that a liar won't be able to look you in the eye. Unfortunately, most liars have been tipped off to this idea, and you may now find that someone maintains a bit too much eye contact when lying to you. Scientists at the University of Utah have found that in some cases tracking eye movements can be more effective than using a traditional lie detector, which measures emotional response. Still, trying to tell if someone is lying based on the direction of his eyes isn't a foolproof method.
Watch the person's eyes as he tells you the story. If he's telling the truth, he's probably maintaining eye contact and occasionally looking to his left -- your right -- and up. If he's lying, he's more likely to glance to his right -- your left -- and up.
Ask a question about the possible lie.
Watch the eye movements after your question. If he needs to make up an answer, you'll probably see his eyes toggle back and forth, as he accesses different parts of his brain to come up with the answer. If he simply looks up and to his left, he's remembering something that really happened.