I have dealt with my fair share of liars. Eye contact is one way to tell if someone is lying to you. That is not always the case because we have good liars that feel no remorse for lying, so they are able to look you in the eye while they spew dishonesty. I dislike those types of liars the most. Most people know lying is wrong and many have a conscious.
Want to know how I tell if someone is lying to me? I make them prove the truth to me often enough to know I will be checking. My oldest son never knows when I might pop up somewhere. If he is not where he is supposed to be, he will hear about it and so will his friends.
I also listen to my gut. If something feels "off" about someone's story, chances are? It's "off".