There are some respected screening tests online, but they are accurate for screening, not for diagnosis.

AQ and AQ-10 are considered validated. The aspiequiz is not (even though I suspect it might actually be more accurate!), and I believe the one on psychcentral.com is not.

I score high on most of the screening tests, but a neuropsychologist doing actual diagnostic tests decided I'm not autistic enough for a diagnosis of ASD. (Granted, it was hard to take that guy's opinion very seriously, but I suspect most other professionals would have ended up at a similar conclusion.)