This is a very setup-dependant trick, meaning that the payoff is something you need to do before even finding a spectator to perform for. Randomly pick a card from the deck (we'll use the Nine of Spades) and place it on top of the deck.
Using your nails, scratch the number "9" onto your forearm/wrist.
You will need to force the Nine of Spades onto the spectator. To do so, split the deck in half and ask which half he wants. No matter what, the top half needs to be used so if the bottom half is chosen, respond with "okay, we'll put this one away."
Force the top card and have him memorize and place it in his pocket.
Tell him to look in your eyes and "send" the value of his card to you. As he does so, tell him you will be able to tell the value and suit by the way he looks at you. Build it up by naming some wrong numbers and suits, but eventually name the Nine of Spades.
Tell him to grab your forearm and think of his card.
When he lets go of your wrist, it will be very red. Rub your wrist until the redness evaporates, and the irritated skin showing the number nine will surface.