Depends on how much information you have about someone, obviously, but one could look for situations with traditional game theory scenarios - may I'm just trying to use fancy words. When I'm in public places I sometimes watch people, trying to imaging (or just taking note) of what kind of person they are - just for fun, there is rarely any feedback but for example if a person goes through some trouble to but a book back on the right spot on a shelf in a library (in a situation where thinks it's not being watched) I take that as quite good correlate for trustworthiness/ability to uphold social contracts.