Any player who has been dealt in to a hand may ask to see any called hand at showdown.
I swear, you can't make this stuff up.
Seen/heard last evening...
4/8 game, progression of hand irrelevant till the end with two players left, good size pot.
player A who was the BB bets out, player B who MP calls and shows a small flush.
Player A folds without showing his hand.
Someone asks to see the folded hand, dealer ceremoniously touches the hand to the muck and then flips it.
Player A goes insane. Dealer explains that anyone at the table can ask to see a called hand at showdown.
"BUT I DIDNT CALL" screams player A ..." I BET"