I've been a bit obsessed with lies and lying at different points over the past years. One thing that I found is that there is a big difference between confronting someone who is lying when i desperately needed them to confess and tell me the truth and speaking honestly to person about lying without needing anything from them. And, actually, a few times, when I've confronted a person in the second way ("I know") without requiring him to acknowledge it, on a few occasions people have come around and told me what was going on.
But the most important thing about it to me is this: am I needing a person to agree with me, to confirm what I know? How paralyzed am I by someone else's lie and how much does it mess with my own perceptions? I am a self-critical person, with a tendency to second-guess myself, but I am learning when to trust my gut without asking for input.