The best way to tell if some one is lying to you is by the way the talk and their body laughuage when they are talking to you.

1) When someone is talking to you listen to teh pitch of their voice and how they say things to you

2) Watch their eye movements, if they are looking down or away from you chances are they are lying to you. Do not confuse this with looking down to the right, if you see this it means that they are remembering what they are saying and getting the facts right.

3) When they are looking away from you or getting really defensive you should ask why they are getting defensive but be careful with this. Sometimes a person goes on the defence if they are telling the truth. If you know the person than you should know how they would normally act.

Most importantly if you are asking a question about something be honest and to the point. Do not stray and get mixed up in the confussion that they are creating to get out of trouble.

If some one is saying that someone else is saying stuff it is always better to go to the source. If that does not help get the source and the other person in the same room and bring it up in a conversation. This is a good way to tell what person is lying.