Hand a deck of cards to the spectator and ask him to mix it in any way. When you get the deck back, tell the spectator that you are getting a feel for a couple of cards that will later be significant.
With the cards facing you, note the top and bottom cards and then look through the deck for their mates. As an example, if the king of hearts is on the bottom, look for the king of diamonds and then lay it on the table face-up. And if the four of spades is on the top, look for the four of clubs and lay it on the table face-up.