In this Alfred Hitchcock movie "Rope" the two guys in the beginning of the movie are killing a guy. One guy is a psychopath and the other is affected, led, in other words, tainted by the illness of the psychopath by not knowing any better. How can you tell? The guy in the brown suit has feelings, the guy in the blue suit has absolutely none. Of course the psychopath will pretend to have feelings, but the more aware you are, the more evidence you can pick up from the rest of them which shows they do not.
Alfred Hitchcock's Rope