However, you at least want to know that the cards you got were fairly given, and provably so.
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.
Cards that you never get to see in a real life game of poker should not be able to be seen in an online version of poker. That includes other people's cards if they don't show, the deck, the burns ... if you wouldn't see it, you shouldn't be able to see it.