You cannot self-diagnose, and only a trained medical professional can diagnose autism. By trained medical professional, I mean an autism expert who is either a therapist (PHD, PsyD, LMFT, etc.) or a psychiatrist.

Most people who use terms like autistic, bi-polar, sociopath, narcist, etc. are using the terms incorrectly.

Also, I suspect you are correct that most ASD diagnoses do not use brain scans and rely on a trained professional's judgement and observations. That fact does not mean that autism does not exist or that some autistic people may have physical differences from neurotypical people.