It only comes from actual experience.
Ya gotta be there.
Otherwise, have a plan to kill everyone that you meet in the most ruthless fashion and as quickly as possible. Keep your head on a swivel and always have a means of escape, never let your self get boxed in or surprised.Watch the eyes and watch the hands. If you cant see their hands then assume that they have something in them.
Thats a heck of a way to have to live, but thats what it has come to in lots of places.