And now let's look at the other side of the coin: what tells you someone's not-so-nice?

Let's start at the extreme. You can generally trust your gut as to whether someone is going to go full-Jeffrey-Dahmer on you :

We then report two experiments in which participants, given a set of headshots of criminals and non-criminals, were able to reliably distinguish between these two groups, after controlling for the gender, race, age, attractiveness, and emotional displays, as well as any potential clues of picture origin.