I just thought of something: in that third season episode where Picard is replaced by a doppelganger, the thing which makes Riker twig that he isn't really Picard is that he leads Ten Forward in a rousing sing-along... something he knows the Captain would never do. Surely when Picard says "Do you know the words of HMS Pinafore?", Worf should be unholstering his phaser and asking "Who are you and what have you done with the Captain"?
