I've spent a long time in disability communities, falsifiability gets very little respect, and "gatekeeping" is generally expressly or implicitly taboo.
So with Autism, probably the most common disqualifier in practice is that the disorder requires childhood presentation and a lot of people don't know this. So somebody will come in, say "oh I wasn't autistic at all as a child, but I felt very autistic as a teenager, so I asked a doctor, and he said I wasn't autistic". People will respond with "oh, doctors don't know anything about autism, they underdiagnose minorities and women all the time, that proves it" and "oh, women actually don't present as autistic in childhood, they can just be masking, but that doesn't mean they aren't actually autistic". This happens EVERY time, especially if a woman is involved, and means that you can't falsify the idea that somebody was autistic in childhood.
I have been banned once for saying that disabled people should doubt other peoples disabilities (on the grounds of preventing exploitation and misallocation of resources), I got told that I was spreading ableist myths that disabled people are responsible for other disabled people lacking resources, that I was reinforcing the ableist myth that disabled people are just exploiting non-disabled people (of course, disabled people do just as much for the able as the able do for them, and thus there is no advantage to faking disability), that it was inappropriate to talk about doubting disabilities on an autism community, and that they wished I had a number of painful disabilities that I couldn't receive pain medication for because I wasn't believed.