Studies have shown that most people cannot tell when someone is lying to them - even someone who they know very well.

There are several theories related to eye movements - that you move your eyes to the right when accessing memory, and to the left when accessing creative thought (aka, lying) - however, these are not really accurate as studies have shown that many people use their creative side to determine how best to tell a story - even one that is true.

For the most part, you just have to watch their body language and their reactions and see if they are consistent with what you know of them.

One big cue that someone MAY be lying is that they get angry at your questioning them.