Who do you know how to tell if some one is lying?
You can know when someone is lying by looking at their body language. You will hear some inconsistencies in what the person is saying, and will tend to accuse you of lying. Check out http://www.ehow.com/how_13299_know-someone-lying.html to learn more.
If there is anybody else involved in what you think was a lie than ask them what happened
If a person won't make eye contact with you, when they become very defensive, when they become accusatory towards you, when they try to change the subject, all of these are obvious signs that someone isn't being truthful.