I recall reading stuff written by security experts about some tale-tale signs that they use to pick out the BG's. It seems one of the biggest ones was what they called "swivel-neck." Someone about to do something bad looks all around to see if they will be spotted, look for cops, etc., as if their "neck was on a swivel." Once you know this one little thing, it's amazing how much easier it is to spot something going down.

Also, take note of the hands. The danger is really in the hands of the BG, so if you can see their hands and keep track of them, it will help. Beware someone who has their hands in their pockets or otherwise out of view.

One thing I do is just look around and take note of who is around me, how far away they are, and what they are doing. It's not hard to do a quick "risk assessment" of everyone in your vicinity with just a quick glance. Just doing that one thing alone shows the BG's that you know they are there and makes it less likely they will pick you out as a target. They are looking for folks who have blinders on and don't see them coming.