When I asked for a referral, and said it was because I wanted to know, my doctor said that I'd never know for 100% certain, and I have to agree with that.
But after a couple of years of hanging around WP after the diagnosis, I'm certain enough that I'm an Aspie. There was no "big bang," I just gradually got more and more evidence until I no longer felt any real doubt. It's useful to ponder what your doubts are, to really think those doubts through. My biggest doubt was that I'd been socially successful for a few years, which didn't fit the diagnosis at all, until I realised that the people I'd been with at that time were very unusual. Put me in the mainstream and I can't cope.
Actually the very fact that you want to be 100% sure is rather an Aspie thing. We're like that, we tend to want everything to be black and white, we're perfectionists. But when you look at the real world, even in a murder trial, they never really prove that the accused did it, they just go for "beyond reasonable doubt," whatever that means, and they hope to god they got it right. With civil cases, they use the "balance of probabilities" which is even more risky. Ultimately, with any question, you have to draw a line somewhere, you won't discover that line has been drawn for you.
Somebody once said that they didn't worry too much about being wrong in their scientific assumptions. They reckoned that it was OK to assume, within reason, and that if they actually were wrong, then sooner or later some fact would come up and tell them that, so it would work out in the end.