I am first to admit my radar is not what it was back when I was carrying a handgun 24/7, but I am mostly on the lookout for odd behavior. Anyone asking for stuff from a stranger that makes no sense - someone asking for a light if they have no cigarette in their hand, asking for the time when they have a phone on their belt or sticking out of their pocket. Out of place, acting, delaying - you can't grow eyes in the back of your head but you can take in odd behavior and movement. Per
@dc, anyone approaching me with big smile out of the blue is a serious attention getter - it creeps me out to see that come rolling up in my direction unless I'm in a store and they have on a name tag.
Some of my old instinct s still programmed - a spare a glance at a stranger's face and then drop focus to the base of their throat. Grooming hands. Small step back with one foot.
Is extremely helpful to look into your local laws as well. Stateside, the regs re use of physical force are pretty standard state to state, but it doesn't hurt to spend a few hours reading appellate decisions. Many law schools now have online searchable databases filled with scenarios and subsequent rulings. You can preempt a lot more often than one might think as long as you can articulate the hypothetical "reasonable man" would have done the same.
I was never a bad guy, but I used to kill a few beers with some from time to time.