Prestidigitation

A question for all you stage magicians out there:

Suppose you truly had the magic power of clairvoyance, and were able to know the order of a pack of playing cards without looking. What would be a good trick to devise such that any reasonable person, layman or expert illusionist, would be forced to conclude that you weren't a charlatan?

To put it another way, magic tricks usually turn on hurdles that the magician sets up for himself, to make his eventual ability to pick the right card all the more amazing ("I'll pick the card... blindfolded!" or "I'll use a brand-new, factory-sealed, unaltered pack of cards") although of course, they turn out to be no real impediment to a skilled magician. Is there a simple obstacle, or set of obstacles, that it is genuinely impossible to overcome without supernatural powers?