I'm a very honest person and I hate being lied to. If I know I've been lied to, I'm never afraid to call the person out on it. I do, however, try to be a little nice about it, although it really depends on the situation. I'll almost always give them a chance to redeem themselves too, kind of dancing around the fact I know the truth and giving them a chance to tell me before I finally let them know in a more direct faction that, hey, I know you're lying.
It is important to recognize that, in certain situations, people may be lying to you because they know you don't want to hear the truth. It may be because they know you're in denial, that the truth may hurt you, or it's just not their place to tell (like if it's a secret someone else is keeping from you). Sometimes taking a step back to recognize just why this person is lying can help you confront them more peacefully or just let it go. Lying isn't always meant to hurt.