2nd I will have a lot of info with me going back to when I was only a baby where I saw some Autism things going on. There will be photos and my 2nd grade report card where the psychiatrist don't have to take my word for it.
Not really the point IMHO.
For my diagnosis, (over 4 x 90 minute sessions), we spent the first one talking about my history. For the final session where I got to receive and discuss the diagnosis, I realised that those 90 minutes were just to observe my social interactions, look for stimming, analyse how I responded to certain questions (e.g.intellectually vs intuitively), what kinds of questions or discussions caused me to hesitate etc. etc. The actual content was fairly insignificant in terms of the final diagnosis. But through this discussion she picked up all the classics: inability to hold eye contact, various stimming movements (very subtle, but she noted them!), a tendency to talk AT rather than talk WITH, to talk in monologues, to talk about facts and information rather than emotions etc. (and a lot more ...).
Alongside that, I had 2 long sessions basically working through tests. One was the RAADS test, then I did two long questionnaires working through various situations and then asking me questions on what was presented: one of these was the Baron-Cohen "faux pas" test and the other was on "intent" in various social exchanges that I've forgotten the name of. Finally, I did the eye recognition test (which I did well at!).
Diagnosis for autism/Aspergers is no different to diagnosis for any kind of condition, medical or mental. You have a certain number of clinical criteria that have to be met. Or not. I was more than happy to work through it with someone who obviously knew what they were doing, and to work through it more or less like a maths exam.
FWIW, I was diagnosed by a doctor (i.e., a medical practitioner, a GP) who became interested in autism and got herself up to speed on diagnosis. Now it's all she does. I appreciated that she came at it from a fairly rigorous, non-subjective point of view which reflected her medical training.