From Roberts Rules of Poker:

If everyone checks (or is all-in) on the final betting round, the player who acted first is the first to show the hand. If there is wagering on the final betting round, the last player to take aggressive action by a bet or raise is the first to show the hand.
Ok, so the last player to take aggressive action shows his hand first. I assume the next person who has to show is the small blind, and then it proceeds to his left?