The key to science is controls for some subjects.
I would imagine that autism is almost unheard of in the Amish and Mennonite communities.
If that is correct that gives us a starting point.
Rate of autism among Amish no different from rest of population, according to studies:
https://www.mastermindbehavior.com/post/do-amish-kids-get-autism
They don’t know why, but Hispanics appear to have lower rates of autism, even after adjusting for socioeconomic factors:
https://pmc.ncbi.nlm.nih.gov/articles/PMC2804636/
There are a lot of Mennonites here where I live. I believe they attend the public school so if the rise in cases was because the schools are more aware and do more testing it would show up among the Mennonite community as well. That could be a good starting point, like you say.
The schools really don’t do much testing though. I’m pretty sure my younger son is on the spectrum. Very, very intelligent. Scored very high on the ACT and can find the answer to almost anything and understands almost everything. But he thinks like a true scientist, and most people - even his university professors - could never understand that and thought he was just “stubborn”. He can be stubborn, but mostly his thought processes are just different. In his case it makes him a genius at programming and specifically at handling security vulnerabilities. The trouble is that he can’t even get interviews for jobs because his professors didn’t understand how his brain functions. In grade school he had a teacher whose son was on the spectrum; she recognized some of the characteristics in my son and wanted him tested. I also wanted him tested but the school wasn’t interested.
I eventually home-schooled him because he was suicidal from a high school “journalism” teacher who wouldn’t let him use scientific studies on his research paper, wouldn’t let him use search engines that would show him scientific studies, and insisted that he use what he knew to be outright dishonest “mainstream” opinion pieces to try to write an honest, factual evaluation of an issue.
Environmental factors were the same for him as for my 3 non-spectrum kids. All were vaccinated as children; that was before anybody was talking about the dangers of vaccinations.