I'm with Val, here.
Well, for a start, a lack of scar on the wrist is very little in the way of guaranteeing you're not going to be dating a psycho. I don't think all people with mental problems cut their wrists. And not all people with scars on their wrists have mental problems.
Secondly, the fact that it hasn't proved wrong so far doesn't prove it's a good method. Ever heard of verification and falsification?
For example, I might say that "I smash an egg on the floor before leaving my house to stop me getting knocked down by a car" and then say "well, I haven't been knocked down by a car yet, so it must be working."
Thirdly, I don't take a lack of a smile from a girl (a guy, in females' cases) I like to be a sign that I should suspect them of mental instability. I'd firstly assume it's a sign they don't wanna smile at me, before trying to check them for scars.