Note the card at the bottom of a deck of cards and memorize it. This is the key card.
Have a volunteer pick a card at random from the deck.
Have the volunteer memorize the card and show it to the audience, and tell him or her not to show it to you or tell you what it is.
Have the volunteer place the card on top of the deck once he or she has memorized it.
Allow the volunteer to cut the cards as many times as he or she wishes.
Look through the deck, starting with the top card, until you find the key card. The one after it is the volunteer's card.