RAY: An off-duty policeman is working as a night watchman in an office building. He's doing his rounds and he comes upon a closed door. Behind the door he hears voices; he hears people talking and an argument seems to be taking place. He hears someone say, "No, Frank, no; don't do it, you'll regret it." Bang! Bang! Bang!
 
The night watchman bursts through the door; what does he see? A dead man on the floor. And the proverbial what?
 
TOM: Smoking gun.
 
RAY: And in the room, are three living people; a minister, a doctor, and a plumber. He walks over to the minister and says, "You're under arrest. You have the right to remain silent."

How does he know that it was the minister that pulled the trigger?