To win any part of a pot, a player must show all of his cards faceup on the table, whether they were used in the final hand played or not. Cards speak for themselves: the actual value of a player's hand prevails in the event a player mis-states the value of his hand.
Because exposing a losing hand gives information to an opponent, players may be reluctant to expose their hands until after their opponents have done so and will muck their losing hands without exposing them. Robert's Rules of Poker state that the last player to take aggressive action by a bet or raise is the first to show the hand, otherwise the first player to the left of the dealer button is the first to show the hand. If there is a side pot, players involved in the side pot should show their hands before anyone who is all-in for only the main pot. To speed up the game, a player holding a probable winner is encouraged to show the hand without delay. Any player who has been dealt in may request to see any hand that is eligible to participate in the showdown, even if the hand has been mucked. This option is generally only used when a player suspects collusion or some other sort of cheating by other players. When the privilege is abused by a player (i.e. the player does not suspect cheating, but asks to see the cards just to get insight on another player's style or betting patterns), he may be warned by the dealer, or even removed from the table.
There has been a recent trend in public cardroom rules to limit the ability of players to request to see mucked losing hands at the showdown. Specifically, some cardrooms only grant the right to view a mucked losing hand if the requesting player articulates a concern about possible collusion. Under such rules, players do not have an inherent right to view mucked hands.
Because the act of folding a losing hand rather than showing it down is so common, some players can take advantage of others who do this with a rare play called a call-bluff. For example, if you know that a player always folds rather than showing his hand if he was bluffing, you might call his last bet even with a hand inferior to the one you suspect him of bluffing with, expecting that he will simply fold before he sees that you don't actually have him beat.