In our system, we have a simple benchmark for determining whether or not you are prepared for a self-defense situation. This benchmark has nothing to do with physical fitness, with knowing a certain number of techniques, or with holding your own in competition, and everything to do with the injurious intersection of intent, physics, and physiology. Objective, self-evident, and brutally uncomplicated, it is simply this:
Are you confident that you can kill a person with your bare hands?
It could be put into more words, but this is the long and the short of it.
Suppose that you find yourself in a worst-case scenario where immediate, drastic action is called for in order to save lives; a situation where, if you had a firearm, you would be legally and ethically justified to deploy it and empty the clip into a human threat until it stops moving, but you happen to find yourself without one. If left to your own devices, if a need existed, would you be psychologically and physically capable of doing the dirty work of a bullet with your bare hands, of wrecking the human machine through catastrophic injury so that it doesn't work any more, of stripping it of consciousness, physical functionality, and perhaps the spark of life itself?
If so, then you are as prepared as anyone can be for whatever you might face.
Obviously, very few situations would call for such a response, and this type of scenario may constitute only 1% (or even less) of situations involving criminal violence, but would you rather be prepared for only the bottom 99% of the threat range, or for all of it? Especially considering that, when potentially lethal violence is called for, it is in exactly those situations where no less decisive a response would be sufficient to guarantee your survival? If you have a concealed carry permit, you obviously know that your firearm is only to be drawn when it needs to be used, and to be used only in a situation where you have no other choice; you don't carry it around so you can mad-dog homies on the street, or bully someone over a parking space. It is the same when your only weapon is your body mass.
Your own death is, of course, merely a proxy for any bad thing you would go to any length to prevent. Maybe they're not trying to kill you specifically; maybe they're trying to abduct your kid, or rape your sister, or burn down your house with your family inside; maybe they have a gun and they're firing into a crowd, or trying to hijack an airliner with a sharpened toothbrush. The law classifies such felonies as "forcible and atrocious crimes", where precedent states that force can be justified to prevent them from happening, up to and including lethal force.
Obviously, there is no safe and ethical way of confirming this benchmark, but that's okay. When you have the requisite intuitive grasp of what makes things break in the human body, when you can not only see with your mind but feel in your bones how to get the work done, it is simply something you know, the same way you know that you could smash a piece of furniture with a sledgehammer, or snap firewood into kindling under your boots, and you don't need anyone to certify you.