Use a deck of cards with asymmetrical backs. Arrange the cards so that the backs all point in the same direction.
Step2
Have a volunteer pick a card at random from the deck.
Step3
Ask the volunteer to memorize the card and show it to the audience, without showing it to you or telling you what it is.
Step4
Ask the volunteer to hold the card out in front of him or her, face down, pressing it with his or her thumb.
Step5
Note the orientation of the volunteer's card as he or she holds it.
Step6
Separate the deck to make room for the card; hold the deck so that it's oriented opposite the orientation of the volunteer's card.
Step7
Have the volunteer slip the card back into the deck.
Step8
Explain to the audience that as the volunteer held the card, he or she left a "psychic thumbprint" on the back of the card that only skilled magicians like yourself can see.
Step9
Go through the deck, looking at the backs of the cards. The volunteer's card will be upside-down.
Tips & Warnings
Even if your deck has a symmetrical design, you may be able to perform this trick. In many standard decks, the border around the design is thicker on one side or end than it is on the other.
One of the most important rules of magic is to never reveal how a trick is performed. If you do, your audience will be disappointed and everyone you know will start doing your trick.