True Lies by Marc Paul

A Lie Detector Test that is extremely flexible and 100% accurate.
The performer borrows 6 business cards from a spectator or, of course, he can use his own. He clearly writes the word "TRUTH" on three of them and the word "LIE" on the remaining three. A spectator is asked to mix them up so that no one knows which cards are which. Keeping the cards face down at all times the spectator now uses the pen to place an "X" on the face of the unknown bottom card. They do this without looking at the card. They then mix the cards again. The Spectator places the cards face down on the table and six people each select a card. The performer explains that no one could possibly know the order of the truth and lie cards and no one could possibly know on which card is the "X". It might be on a truth card or on a lie card. The performer says "Imagine that the six of you are suspects in a murder enquiry. Just like in real life you all have your own agenda. Some of you will be completely honest and others amongst you will lie. The card you hold has randomly assigned to you what character you are. HOWEVER, the person with the X card is our murderer and they can choose to ignore what is on their card and make up their own mind about whether to lie or tell the truth."
The performer now asks one of the spectators a simple question. Like "Are you the murderer?" or "Did your card have an "X" on it?". The spectator answers "Yes" or "No". Immediately the performer can tell if the spectator was lying or telling the truth. He can repeat this with any of the spectators. He is 100% correct every time. As a climax he can even reveal whether the murderer is lying or telling the truth and who exactly the murderer is!
Points to consider...the business cards can be borrowed and are not prepared in any way. The spectator does all the mixing, draws the X, mixes again and then places the cards on the table. The six spectators have a free choice of which card they pick up. Believe it or not the performer genuinely has no idea whether a person is a truth teller or a liar until they respond to the question!