Four players are about to take a flop when Player A accidentally exposes part of his hand to Player B by being careless with his cards. Player B states that he saw the 7. The dealer and floor want to inspect Player A's hand and flip over the 7 if it is in fact there.

What is the proper ruling?

There is no specific rule to handle this situation in most rulebooks.

In general, I prefer just announcing "player reports seeing the 7s" to put all opponents on even ground whether or not he was correct.

Some floors will require you to open the exposed and identified card (if you have it) to the table. I don't like this as much, particularly when you don't actually have that card, as now your holding is narrowed unnecessarily.

Player B states that he saw 7s and all other players choose whether or not to believe him.

I would be fine with letting him play with one exposed card, then after the hand ask him to be more careful, because it could result in the hand being killed next time.

A lot of rooms don't allow players to expose cards.

That is really horrible. What stops players from trying to play Go Fish?

That's why he doesn't like it.

Accidentally exposing your hand should never lead to having your hand killed.

For that matter, purposefully exposing your hand should never lead to having your hand killed.

All rooms should have a rule against exposing your hand (unless heads up). But the punishment for purposefully breaking the rule is to be 86'ed, not to have your hand killed.

This is to me potentially a problem.

Player B could be deliberately lying and gets the advantage of knowing what he saw and knowing what his lie is.....

I think these situations are pretty touchy but the more obvious it is that the hand was exposed the more likely I think the proper response is exposure of the card.

A player saying he got a quick glimpse of the card should not mean that we expose a players cards. But if their is no question at all that the card got exposed then I think the burden of this has to fall on the player who exposed their cards rather than on all the innocent players at the table.

So I tend to favor an approach of looking at each circumstance and trying to fit the best solution to it rather than a cookie cutter ruling

Okay. What additional information would you like from OP?

The nature of the exposure is important. Is it a player claiming he got a flash of them? Or is this a player who was holding them up carelessly? Dropped em? Was it intentionally shown (guy thinks he is showing them to a player out of the hand)? Does the player whose card(s) were exposed agree that they were exposed? Is there agreement that they were exposed? Is this one of those situations were neither player in the "exposure" believes they were exposed but a guy at the other end of the table is certain that one player could see the other players cards. Does the exposure have a history of exposing cards or handling them carelessly?

I'm not asying I need OP to tell me this. What I'm saying is I want a floor person making a decision to have this kind of information to tailor the ruling to the circumstance.

I don't a player to have to play with a disadvantage at all. If it can not be avoided then i want the disadvantage to go to the player who is at fault. But I'm not going to expose a players cards over "I think he might seen......"

If the player who is alleged to have exposed the card agrees it was exposed that will almost always be enough for me to expose it. The bigger problem is when there is a factual issue about whether it was exposed.