Before the trick begins, take a 9 of any suit, say the 9 of Diamonds, then rip it so that there are only five diamonds on the actual card.
Place that torn half in your pocket.
Shuffle and cut the deck, and either through false shuffles or other techniques, ensure that the 5 of Diamonds, or the 5 of whatever suit the 9 is, is on the bottom of the deck.
Have the spectator cut the deck and place them across one another, meaning the 5 of Diamonds is on the bottom of the top half.
Ask him to pick a pile. If he picks the 5-Diamonds pile, keep it. If he picks the other one, say you're scrapping whichever he chooses.
Tell him to take the bottom card and memorize it. Then say you have the same card in your pocket.
Pull up the card from your pocket, not showing that it is torn, and say it matches. He will see the 9 and argue that it's incorrect.
Pull it out and show only 5 diamonds are on the 9 of diamonds and you were right. Everyone will laugh and then realize you knew he chose the 5 of Diamonds!