Well said - QFT.I don't have to prove what someone is thinking. I can only act on, and can only testify to, what I can observe and conclusions rationally drawn from those conclusions. If I square up with you, crack my neck, ball my fists up, raise my hands into a boxer's stance and start closing with you those are all things you can see. Those are all things you can testify to seeing. A rational conclusion based on that body language is I am getting ready to punch you. If I threaten you with a pistol and have the weapon in my hand, its not legally required to guess if I'm bluffing or not. I don't have to take the first punch in the first scenario any more than I have to take the first shot in the second just to see what you were thinking. A reasonably prudent person in your hypothetical situation would believe an attack was imminent.
Now again, I think if you shoot someone for an imminent attack of a punch, you're going to have a steep uphill battle to justify it as reasonable.