From Robert's Rules of Poker, Version 11:
"5. Any player who has been dealt in may request to see any hand that was eligible to participate in the showdown, even if the opponent's hand or the winning hand has been mucked. However, this is a privilege that may be revoked if abused. If a player other than the pot winner asks to see a hand that has been folded, that hand is dead. If the winning player asks to see a losing player’s hand, both hands are live, and the best hand wins."
Bob Ciaffone's website:
http://www.pokercoach.us/
Quote taken from:
http://www.pokercoach.us/RobsPkrRules11.mht
His website also offers a link to his rules for home games.
Note that the quoted text from above is from the newest version, Version 11. At least some previous versions, as refelected in his link, "Web-based Robert's Rules of Poker," show a slightlly different version of the rule. It confused some readers because it referred to "
any hand that has been called" when he really meant what is reflected in the newer version, "
any hand that was eligible to participate in the showdown."
From Version 10:
"5. Any player who has been dealt in may request to see any hand that has been called, even if the opponent's hand or the winning hand has been mucked. However, this is a privilege that may be revoked if abused. If a player other than the pot winner asks to see a hand that has been folded, that hand is dead. If the winning player asks to see a losing player’s hand, both hands are live, and the best hand wins."