Hi JGG,
When I was a kid I spent part of my time living in rough area of my city. I had to calculate the best route to school in order to avoid gangs and some local thugs. Unfortunately, I didn't always succeed and was occasionally beaten up and once even had a knife held at my throat.
I know thugs. For me it's a case of you can "sense" them. It's something in their body language, their eyes, the way they watch you, like you are prey (which you are to them). It's the barely perceptible sneer, the dead eyes, the visceral feeling of animosity when they are near you.
It has little to do with how they are dressed. As a young man I was accosted by two thugs and was unable to stop them. It was only the intervention of my mother which prevented it getting worse. The thugs in question were both uniformed policemen. They had the same sneer, the same dead eyes, they were just smart enough to say they were looking for a suspect or something. They soon vanished when they realised their pretext was flimsy.
Another time one of the biggest, scariest looking kids in our area protected me from a gang who had got hold of me. They were all scared of him and let me go. He walked me home like a guardian angel. I'll never forget that.