You can only be as sure as you allow yourself to be. Nothing can be proven when it comes to metaphysics.

Apologies for not catching this in a more-reasonable amount of time, but you're going to have to be more specific about your question. The vast majority of humans throughout history agree that they are "someone", though the full nature of what "someonehood" entails is definitely debatable. I assume you're asking some question about the full nature and definitions and whatever, but you're not really doing it in a way that's answerable. If you can quantify that question better, I can attempt an answer. But this question is the epistemological equivalent of "how fast is a car?"