Ask them about what you think they lied about earlier. then they'll have to think about what the lie was, or then they'll say, "I don't remember talking about that" and then you'll be like "BAM! YOU A LIAR!"
The lies people choose to tell can be just as revealing, or more so, than a factual statement. Textbook example - a person who is obviously much older than than the age they say they are, is actually telling you that they are afraid of getting old. And that's the truth. If we know how to really listen, to read between the lines, and to remember that we tend to believe what we want to believe - we are not so easily misled.