I can tell you how to tell afterward. I've had people tell me in post shooting interviews they missed on purpose. This was such a foreign concept to me that I can barely begin to describe the psychological pressure that was in my head to not stop the interview, slap them, and then resume the interview. Your life was in danger and you decided to use your gun like a starter pistol? You just relied on fear to keep him from shooting you and fleeing instead? God favors fools and drunks, but come on! I've sure others who don't actually admit to it have done so as well based on their body language and responses during the interview.
I think many people have a gun but don't have a plan. They think the gun makes them safe. It is a talisman against evil and simply needs to be carried, not a tool that needs to be partnered with a plan and implemented. They never play the "what if" game. I'm a firm believer in running scenarios in your head. Me and my partner were at a restaurant with a very small dining room. A guy goes to the bathroom, a 1-holer right with a door about 2 yards behind my partner's chair. "What if" time. "What if he comes out with a gun to ambush us? I formed a plan. I was then vaguely surprised when he came out and went about his day. I'm vaguely surprised a lot. This is good. My first FTO told me my first traffic stop, expect them to run. Then when they stop you're pleasantly surprised. When they do run, you aren't surprised at all because you expected it and you'll execute your plan.
If you can convince someone of the value of the "what if" game, and if they do it, and if that plan involves shooting if necessary, I would hazard that its more likely than not they will shoot if required. I firmly believe that with exposure and training, most (not all) folks can reach the point where they can.