When you begin your performance, spread the cards showing they are ‘random’, then ‘notice’ the Joker, take it out and split the deck at that point. Give one half to each spectator. They do their selecting and swapping, but when you get the cards back you simply look for the card that’s out of place in the respective sections. There will be a prime valued card in the non-primes pile and via versa. Use your performance (and mathematical) skills to reveal their selected cards. You could present this as a demonstration of your super-powered sense of smell. When the spectators.
