The most important thing is to be honest and truthful. As questions are asked, don't get nervous, just answer them truthfully. If you have had painful moments in your life, and you are asked about them, don't ignore them in the office---talk about them. The doctor needs to have a complete view of your life along with its challenges. Something you might do is to compile notes on your life---from early childhood to however old you are now. By compiling notes, this can help the doctor. If you are like me, I sometimes goof up a bit when I try to communicate verbally, so having notes on my life helped both the clinician and myself.
As to what to expect---it can vary. My diagnosis came in a relaxed office in Cincinnati, Ohio. My son's diagnosis came in a more typical psychiatrist's office where the doctor sat behind a big desk. Basically, the doctor will talk to you and utilize diagnostic criteria. You may or may not be given tests to complete. The main thing is---there is nothing to get nervous about. I look back on my diagnosis appointment and found it to be comforting. It is was really an interesting experience. Getting my diagnosis of AS gave me a new journey in life---a journey of understanding my eccentric life. I went through some therapy afterwards. My son now has an IEP which has helped him tremendously in school. So think of positives that may come from your appointment.
_________________
"My journey has just begun."