When he neither sighs, groans, shakes his head, nor weeps in response to any player during the game.
When the party is speculating on what might be going on and he exclaims: "Ooooh, that's much better! Lets go with that instead."
When he carves his girlfriend's name in dwarf runes on his chest with a razor blade.
When he has to compulsively scrub his hands each time he consults his copies of the player sheets.
When he carves a comprehensive list of famous girlfriends from history, literature, and the lively arts--with extensive notes on primary and secondary sources--in dwarf runes on his chest with a razorblade.
When you inexplicably hear the sound of dice rolling each time he sneezes.
When he plays a tape of a game session to his fiancee because she was curious about what they're like...the one where a very, very tired player decided to start casting Enlarge on various extremities of innocent peasants while quoting Monty Python.
When he claims to have Monte's secret project "in his pants." [Somebody was bound to say it, I just wish it hadn't been me...]
Two of the above examples are true. One other was inspired by reality.