The performer shows one face up card and one face down card to the spectator. After naming the face up card he then turns over the cards and names the new face up card. Now, the magician takes the current face up card behind his back and asks the spec to name the name of the card that should be in his hand face down. He does so and the magician states that he is wrong and proves it by bringing out the card he named from behind his back. Wonderful trick, only it requires gimmicked cards, which are easy to make/buy for almost no cost. However you could have un-gimmicked cards to switch them for after the performance so everything could be examined.
The tutorial also explains how to make the gimmick cards at home which is very simple.
This trick is suitable for all levels.