The original diagnosis was done at a children's hospital with: child psych, dev ped, OT, PT, SLP. And a whole bunch of intern/residents were there watching. The actual diagnosis was from the psych, with input from the rest of the team.
They ran a bunch of tests. I grabbed The Notebook here so I could list 'em for you.
first eval at 2 1/2:
Vineland (given to Mom)
medical history (from Mom)
CAT/CLAMS
speech: Rosetti, Receptive-Expressive Emergent Language Scale
audiological assessment
That might not sound like so much, except that they also spent a ton of time "playing" with him too.
Second eval at 4 1/2:
Vineland
Weschler (preschool)
Autism Diagnostic Observation Schedule Criteria
CARS
PLS-4, Goldman-Fristoe Articulation, Checklist of Pragmatic Behaviors
Developmental Test of Visual-Motor Integration
ADHD rating scale (he also has an ADHD diagnosis)
The last two re-evals we've done they've ran the WISC/Weschler on him. First time he was too young. I don't know that the WISC is any indicator of autism, other than showing his verbal vs non-verbal, but it was interesting to see the results.
The last re-eval we went to, just about a month ago, they only did the WISC and a speech test, and the psych did observation and got history from me. We didn't get the Head Dev Ped and the Head Child Psych this time

(we've been downgraded? LOL) and we also got a different diagnosis. Slightly different.
One of the more frustrating aspects of testing for autism is that there
isn't a good test, really. That's why they did so many, combined with observation. Last re-eval the dr was explaining that to me, which I already knew, and we were joking that one of us needed to invent that *perfect* test for autism. So then we'd be really really rich.