My own personal litmus test (I'm there, comment #77) would be the absence of books. I am fairly stunned when an individual doesn't own a single book. I don't know that I've ever gone back to the apartment of un amoureux and found it bookless, but I did once have a roommate who didn't seem to own any books (actually, I think he had one, but I'm not sure if that was just a gift or something). I find this creepy.
So, how about you, three faithful readers? What discoveries in a potential mate's apartment would send you sprinting for the door?