Any card that isn't specifically mucked by another player could be audited... I think. So that would be: the players own cards, the burn card, the flop, turn, and rivier cards (maybe), any card that makes it to a showdown, plus any remaining undealt cards in the deck. That's quite a few cards that could be audited. The audit wouldn't be airtight, but I see it as a step in the right direction.