Collusion and/or soft play are a tough thing to prove. One of the few undisputable times in which a player is softplaying is if there is no one left to act after him and he checks the nuts.
Something to keep in mind is that it certainly is possible that a player may have misread his hand, other times when there is one card to broadway and already been bets on previous streets that the last player might check figuring the other player wouldn't have called him without having same hand anyway. While both of these cannot excuse the infraction and should still be grounds for at least a warning, just keep in mind that it's quite often not collusion, per se.
As an aside, I have always installed the rule that it must be the exclusive nut hand to warrant a penalty, meaning the nut hand without the possibility of another player tying it. I have done this for the reason that a person could argue that he was a skilled enough player that he knew the other player had the same hand as him and betting was a waste of clock time.
The excuse that a player checked the exclusive nuts just so that his opponent has to show his hand, or show his hand first is not a valid excuse.