Hmm. I'm not so convinced about the waitress test. Obviously, if someone fails it, that's it, and you should get the hell away from them asap. But my ex passed the test with flying colours. He was always so charming, over polite and friendly with waiters/waitresses/bar staff. But he was horribly emotionally and verbally abusive in private. It was all about showing what a GREAT guy he was in public. So I'm not sure you can rule someone out just on the basis of how they treat restaurant staff.
I actually think my ex treated staff so nicely as he was, in his head, patronising them. Like he was deigning to treat them as human beings, because he didn't actually seem them as such, if that makes sense!
Woe betide anyone who he had to speak to on the phone though. I was constantly astounded by how utterly horribly he would speak to people in call centres when he had a complaint or query they couldn't answer. Sometimes he would literally scream at them, and would tie them up in knots just so he could enjoy complaining about them to their managers.