Ask the spectator to spread a pack of cards, which they have shuffled, face down on the table. Tell them to remove a card from the upper part of the spread, note what the card is, and place it in the lower part of the spread. When the card is inserted into the lower part of the spread, make a mental estimate of how far the card is from the end of the spread.
Gather the cards and put them behind your back. Count off to within four cards of the estimated position, and put them on top. Take off 4 cards from the top and one from the bottom and ask if the card is among the five. If not discard them and repeat. When the card appears you know it is the one drawn from the bottom.