I have recently been using a perfect test on how to tell if my friends are lying. Believe me it works, I even used it to correctly predict that my friend had a Q 7 in a game of poker.
What to do:
Just casully start talking to them about a random topic, say oranges, just to keep them relaxed. Then move in closer and ask them "what did you have for breakfast?" NOW WATCH THE EYES.
They will either move to the left or the right, make a note of which way they go. Then ask them the question you want to know if they're lying about "did you sleep with my boyfriend and dad?" Then watch the eyes again. Which side did they go to?
If the eyes go to the same side both times: Telling the Truth
If the eyes go different ways: Lying n00b
When being asked a question the brain either consults the Memory bank or the Creative bank. This is shown by which way the eyes move. Normally if they go to the left (looking at the person) its the Memory side. But its always better to ask the question before hand just to confirm it.
Try it out, see if it works. And post back results.
As mentioned before my best one was using it to get the Poker hand and dodge out of a pot I was heavily comitted into
Note: If your friend is a chronic lier it may not work. Or if they are embarrased about what they had for breakfast.