I don't know why ppl think pro diagnosis (dx for short) will make you know 'for sure'. I have one, I was dxed by an autism and ADHD expert. He said it was 'quite clear' that I have ASD, but it wasn't his opinion that gave me confidence. rather, it was the research and thinking and writing I did beforehand, preparing for my assessment, trying to figure out whetehr the label fits.
as part of that process I made a 9 page document where I listed my traits with examples, the different sections organized according to core DSM criteria first, then several sections that go beyond clinical criteria.
if there is interest, i could share the doc on WP so that people questioning whether they are on the spectrum have a profile to refer to for comparison. naturally autistic people can present very differently but there are core traits supposedly common to us all that are necessary for a pro dx. whether these are themselves correct as criteria is a different question of course.
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personal autism blog.
twitter @zettelstelle
Dxed with Autistic Spectrum Disorder 16/04/2016
Formal diagnosis is for access to services, not to validate me as Autistic. Self-diagnosis is valid.