Well... this kind of depends. In Tourney play, you must show your cards. In a cash game, this is where it gets a little tricky. If you go to showdown, you can request the dealer to show the cards of the player who mucked. However, I would assume you are playing "cards speak" and if the player mistakenly threw his winner away in error, the he will win the pot. If the player requesting to see the hand, must declare to the dealer that his hand is dead prior to show (this is down when the dealer taps his cards in the muck).
For the most extentive list of rules, visit The Coach (Robert Ciaffone)
http://www.diamondcs.net/~thecoach/RobsPkrRules5.htm