Well I'm guilty of using my peripheral vision when someone is behind me so I can check them out without being to obvious. Clenched fists of someon approaching is usually a good sign. Alot of cops have tatoos but most departments dont allow them to go below sleeve length, slightly above the elbow. I guess the biggest point you make is it can be anyone. Nowdays everyone has a gun even kids, and Ive seen old ladies and other women who conceal knives and arent afraid to use them. So it could be your typical gangster or the crazy old lady that missed her meds that morning. Usually you can tell if someone is just trying to get attention or they are looking for a fight. Watch out for the quiet ones, theyre probably going to rob you. The best way is just not to put yourself in the position of being a victim. Dont walk down any dark alleys alone, if you know what I mean. The best defense is surprise, if you notice someone following you or staring you down, dont let them know you noticed. Most of the time they're looking for an easy victim, they dont want to make it difficult. Hope it helps.