It's true they tend not to look you in the eye, but watch for this: the liar will almost always look to his left when he is lying. this is a dead giveaway.
The only liars who can get away with it routinely are psychopaths.
I agree with SM, and want to add if you truly WANT to learn if someone is lying (many people don't want to do that, they want to ignore it or only look to discount that they are lying) then believe what evidence you find that he/she is lying.
Believe the evidence. Without insisting on more and more evidence. Many people reach a point where they say I'll only believe it if I hear an admission from the liar himself, because they don't want to acknowledge the lies.
So it's up to you - your choice - if you want to know when someone is lying, believe the evidence when you find it.
Often they don't look you in the eye. Often they have a straight face no matter what the subject matter. Often you will have a hunch that they are lying. Trust your gut. Mine is usually right. good luck