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Many of magic's classic effects involve what magicians call a 'key card'. If you know the value or location of a key card, you can reveal a spectators chosen card without actually knowing the spectators card. Pick up a deck, spread it, and ask a spectator to select a freely chosen card.
As he is looking at his card, catch a glimpse of the bottom card.
Lay the deck on the table and ask the spectator to put their card on top of the deck. Now, ask them to cut the deck once. Since you know the identity of the bottom card, you can now find the spectators card. Cutting the deck once places the former bottom card on top of the spectators card. You can now cut the deck several times yourself and the two cards will remain one above the other, with the spectators card always one below the card you glimpsed.
You can now look through the deck and disclose the spectators card, or, flip the cards one at a time, looking for your key card to show, and you'll know that the next card will be the spectators. The ONLY exception would be if you somehow cut directly between the two cards. If this happened, and you didn't see your key card until the very last card in the deck, then the spectators card will be the very top card of the deck. They will have returned to their original positions.
You can use this technique to discover the spectators card and not actually reveal it. Look through the cards, find your key card, spot the spectators card below it and memorize their card. Now, you can reveal it in any number of ways. You can state that you're unable to locate their card as you thumb through the deck. Handle the deck loosely and separate it between your key card and their card. Informally cut the deck and bring the spectators card to the top. You now have the spectators card back on top of the deck and you can perform dozens of different 'reveals'. Be creative.