Spread the cards face-up showing that they're all different. Ask your spectator to point to any card and you'll tell them which card they picked...

Pause for a second while they get the joke and say, "Oops... that's not real magic... let me do it in the dark."

With that you flip the deck over and spread the cards from hand to hand. Ask them to pick a card now... this time there's no funny business and you know exactly which card they picked!