Devil Divination by George Blake

A pack is thoroughly shuffled (it may even be a borrowed one) and a spectator merely thinks of a card as they are shown to him one by one. Immediately the card is thought of, the pack is cut again and again. The spectator is given the pack, in two halves, to confirm or otherwise, that his card is there. Whether he says it is or isn't in either half, the performer knows the card immediately and may disclose it as he thinks fit.

No questions asked. If you can cut the cards you can do this. Inside two minutes you know the card the spectator is thinking of and you can build up the effect into a complete miracle.