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 :

Now I don't want you stereotyping people or thinking you're ready to be a criminal profiler . But if you really want to know if a man is dangerous, ask a short guy :

Although men generally perceived masculinized faces and voices to be more dominant than feminized versions, this effect of masculinity on dominance perceptions was significantly greater among shorter men than among taller men. These findings suggest that differences among men in the potential costs of incorrectly perceiving the dominance of rivals have shaped systematic variation in men's perceptions of the dominance of potential rivals.