Welcome! I was diagnosed last year at age 29. I score on the higher side of functioning/low support needs for certain.
There is no 100% surefire way of knowing you're autistic for sure, though I find studying the DSM 5 criteria to be very helpful. And a formal diagnosis can give a feeling of validation if that's what you need.
Not with autism, but I have in the past struggled very greatly with imposter syndrome. For me, it fell into an OCD pattern where I was constantly going into thought loops and fighting my thoughts with the compulsion to "check" my questions over and over.
I think the main reason I didn't struggle in that regard with discovering my autism, was that by that point I had been on meds for OCD for a few years.
_________________
He/him or they/them pronouns, please.
ASD level 1 & ADHD-C (professional dx), dyscalcula (self dx), very severe RSD.
Recovering from autistic burnout.
RAADs: 104 | ASQ: 30 | CAT-Q: 139 | Aspie Quiz: 116/200 (84% probability of being atypical)