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.
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ASD level 1, ADHD-C, most likely have dyscalculia & BPD as well.
RAADs: 104 | ASQ: 30 | Aspie Quiz: 116/200 (84% probability of being atypical)
Also diagnosed with: seasonal depression, anxiety, OCD