After a player has been declared the winner (4 Aces) after the other player declared he had a flush, the first player folded her cards and the other cards and passed them to the next dealer. After that the other player declares he not only had a flush, but a straight flush, is it too late or should he then be declared the winner?
If other players also saw a straight flush, you can, depending on if you can trust these players.
I might try to see if his hand was still together in the muck and try to verify it that way.
But if its only the player that had the flush I would not, its tough luck for them, figure out what you have while it is face up. They were not sure what they had while they were looking at it, they are even less sure now.
Players do try to angle "this oh I actually had something that beat you after the hand is mucked". It falls under protect your hand, and you lose if you don't. Were I am at they do not hold up the game for it, and they do not look at video unless the dealer screwed up and mucked the hand without the player conceding it. Since it appears you pass the deck, everyone should be protecting their own hands, if they don't, oh well. It is always better to protect the game rather then to protect a player from themselves.
The standard rule in most casinos is that "cards speak", meaning that when players' hands are properly tabled face-up for all to see, it is the obligation of the dealer, other players, and floormen to ensure that the pot is awarded to the best hand shown. What players say is irrelevant.
If it can be determined (perhaps by agreement of other players, looking at discards, or by looking at camera footage), that a player did indeed have a straight flush that was properly shown, the floorman should award him the pot, even if the dealer pushed it to the wrong player. But there are some caveats: if the hand was not properly shown to all (say it was just shown to another player), the player is out of luck. Also, as a matter of pure practicality, if the issue is not raised right away while it is still possible and reasonable for the floorman to do something about it, he may well rule that nothing can be done.