Couple of things.
1) I was assessed privately, this can be expensive. My first assessment was with my therapist at that time and the clinical psychologist.
2) My second was with my parents and the clinical psychologist. Are your parents alive? You will need them to provide your childhood history - anything about your language, speech at a young age, what you were like in school, how you related to peers, if you had friends, etc, etc. Ideally, this should be done in person rather than you giving it second hand to the psych, via your parents.
3) This is an area where some snake oil salesmen operate, unfortunately. You need to find a good practitioner/diagnostician. They should be adhering to best practice
guidelines.4) The assessment can be intense and draining. Take some time for yourself afterwards.
5) Tests and assessment tools are generally not available online as they cost a bit for the psych to use/source. iirc, the
Autism Spectrum Quotient was one used in my case, I'll have to revisit my report to check.
6) There is a public perception that the issue is massively over-diagnosed (ADHD has the same negative image), but this claim OTT. It is harder for women and girls to get diagnosed due to the lack of awareness at the clinical level and because their presentations can differ from men and boys.