I went through basically (for PDD-NOS and autism diagnoses, multiple times for multiple doctors, so not necessarily all of these at once)...
...history taken from my parents (without me present or aware of what was going on -- first thing they did, which is what first gave them that suspicion).
...weeks and later months of observation in an inpatient psych setting (I wouldn't recommend that).
...seeing me on a regular outpatient basis.
...lots and lots of testing. IQ testing not the half of it: Tons of other tests, designed to show a lot of different aspects of brain functioning and patterns of cognitive skills, as well as things like the ability to parse sounds into words and words into meaning, tell visually which drawings of objects are the same or different (such as a coat folded into different positions), etc.
...interview with me and/or my parents/family and/or my staff about my current life and past history.
...several different kinds of brain scans.
...attempts to rule out any and all other causes for what was going on, including medication trials, etc.
...consultation with doctors who had either (a) previously diagnosed me, (b) known me since childhood, or (c) both of the above.
However, those diagnoses were much more involved than most people I know, I've been surprised at how little observation/testing most other people get.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams