Here's what I say. I know this can sound like comparing apples to oranges, but my paternal grandmother was telling people, in 1999, that she had diabetes. She couldn't eat this or that because of her diabetes. The truth was, she had a high random glucose reading and to be diagnosed as diabetic you have to have a glucose tolerance test, and if that is high, you have diabetes.

Most tests for autism are geared towards children, and some therapists are not knowledgeable about adults. If you do every test in the book and it all says the same thing, you can be reasonably confident. Someone else here made a comment about taking things from others, but it's not like borrowing a disabled parking permit from your friend so that you can park closer to the supermarket.