When asking your #autistic friend how they figured out that they are, in fact, autistic, keep this in mind:
Autism is defined as a collection of traits. Each trait, when viewed in isolation, can seem perfectly innocuous.
So when your friend gives an example of something they struggle with, you might be tempted to say, "but I do that too, and I'm not autistic" or "but everyone does that".
Your friend is only sharing a small fraction of the long list of ways in which they're different. That list is always very long, and kind of personal too. It's also difficult (and sometimes embarrassing) to convey the intensity of each item on that list.
Autistic people who weren't diagnosed in childhood frequently struggle with autistic imposter syndrome, and your kindly meant "well, everyone's a bit on the spectrum" or "you don't seem autistic!" can be painful and invalidating.
@Zumbador I'd say that this even extends to autistic folks talking to other autistic folks.
"Autism" is a bit of a catch-all for a lot of different ND things (and the line between it and ADHD and some other things is drawn strangely, but that's a different discussion), so one autistic person's brain/experience does not necessarily look like another's.
@joepie91 absolutely. And essential for us to remember that our own experience doesn't define what is true for others. Absolutely.