Question: If you call a bluffer's bet on the river in Hold'Em, how can the bluffer muck his cards?
Answer: The bluffer is completely free to muck his cards and concede the pot in Hold’em Poker and there is nothing you can do about it. Except for a few unusual circumstances, the poker players can see the cards that are mucked by the bluffer. In my opinion, asking to see the cards in these circumstances is a big mistake. Firstly, if the bluffer has misread his own hand and has some other good hand and if the player who is "conceding" has not said, "I fold" or done something else that kills his hand, your curiosity could cost you a pot every once in a while. Secondly, when a person mucks without showing, you already that he has a bluff. And asking to see the cards at such a gaming stage might only irritate him, making him to put in more efforts in the poker game to play better. Also, this will make him less likely to bluff too much. If someone else mucks his hand when you call, I will suggest you to take the pot and be content with it.