The Quest/Waypoint tab allows you to view all of the completed quests and waypoints for each character for each difficulty level without having to open the character in a game. Choose the difficulty level from the drop down list on the bottom (Normal, Nighmare, or Hell) and choose the Act from the tabs at the top. It works just like the Quest and Waypoint screens in the game.
The quest page also indicates if the Cow King has been killed. Look in the lower left for the text "* Cow King is dead".