I find it rather interesting, from a social experiment aspect, that people who are going to grief/kill you out of the blue will act sketchy or odd. They'll talk to you. They'll say weird things. They'll ask you where you're going, or just random questions. They'll ask for help, or to group. Make noises or sounds. Play music. The people who are going to do harm are the guys who don't just pass by.
I often say 'yo' when I run into people, indicating that I see him, and they are aware of me. Nothing more, nothing less. If they say 'whats up' or some other simple greeting, 49 out of 50 times, they're cool. It's the guys who draw attention that puts me on edge. The guys who say "Dont shoot!" or tell their members "Hes cool leave him alone" that I know I'm about to get plugged in the back.