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Two thoughts on this. Note that I'm not explicitly disagreeing to you, just responding.

(1) No blood marker, no brain scan, true. But, it's something that does become evident in people later, so the test has later symptoms that it can be checked against for its error rate.

(2) It could be "hysteria", but it's still people acting unusually (at least later in life). Perhaps it's something that's changed in the way we raise our kids, or environmental, or changes in prenatal/birth-giving/etc practice. Any way, it's a change in our population and is interesting enough to follow.

Perhaps at high-functioning end of the spectrum you can get plenty of misdiagnosis, but certainly for the bulk of autistic people you can tell that by the age of 3 when a child simply ignores everyone around them as if people around them don't exist, can't answer simple questions, can't express needs, that something is really wrong.

Autism spectrum disorder diagnosed using a standardized "observation" (the ADOS) and/or a highly structured parental interview (the ADI-R). There are peer-reviewed papers which report on the validity and reliability of these instruments.