Outside of a couple of sizable errors, I've generally always had a good vibe sensor. Most bad people have an approach to things that gives them away if you're sufficiently cynical. The obvious things I've used:

1. How do they treat service staff? This is a HUGE giveaway that most of the bad faith actors don't even realize gives them away.
2. Watch their eyes when they think they're not being observed. WIndow reflections and such are great tools for this. Where are they looking at me? Are they looking at me or the cute waitress when my head's turned?
3. Listen carefully when they speak. A quick calculation of what proportion of their speech is grievances over things done to them speaks volumes.
4. How do they cope with disagreement? Even the best social chameleons tend to fail at this one.
5. Do they even ask about me?
6. Do they listen as well as speak? (And by "listen" I don't mean "wait for their turn to speak" either.)

These are some of the obvious ones. Less obvious (and harder to communicate) ones include a whole lot of body language.