Show one show all. (Robert's Rule?)
Note: ‘all’ refers to people, not cards. IE. If only one card was exposed, only that one card gets shown.
But there has to be a judgment call on who saw the card. Assuming the only people that saw the card we’re already out of the hand, then you can show it, but only once the hand is over. If anybody still in the hand saw it, then you immediately show it to all.