I mean, you could go to a neurologist for objective proof. Although I'm curious as to how you say you're autistic yet your parents don't believe it's real if you don't have a professional diagnosis, and if you don't have a professional diagnosis, how you can say you have autism. That's the only way you know for sure. I got a diagnosis at the age of 2 since I had severe developmental delays, but the tests nowadays can show positive later in life, but again, you need to go to a neurologist for it.
Assuming you have a diagnosis from a neurologist, and they say it's not real, well then there's just no hope for them. I figured out a long time ago that people will not accommodate me, and that I'm the one who has to accommodate everyone else. It sucks a lot since the only thing people need to do is simply have awareness in many cases, but, that's too much effort for them so people like you and me get to be the ones putting in too much effort for others.