The first inkling the professionals had was just a conversation when I went in to get prescriptions for meds for depression... I had mentioned some things about my childhood, and how my mom had recently revealed to me that they had wanted to label me autistic when I was eleven. That started the process.
Eventually I had a full neuropsych evaluation. Half an hour's interview with a psychologist, some tests of memory, attention, sensory processing; an IQ test; some personality tests including the MMPI, which is probably one of the most boring tests in existence. I also had a vocational evaluation, which included things like testing academic skills and doing some tests that helped rate what my career goals and abilities were.
Not the end of the world, but it's new, unfamiliar, and you have to think hard; so you'll probably be pretty stressed out by the end of the day. I pretty much just flopped into bed after we finished.
Far as I can tell, this is on the more thorough end of things. Some psychologists might just do the interview and nothing else. Depending on your situation, that might be all it takes, especially if your case is fairly obvious. In my situation, they wanted a more complete picture mostly because I was trying to convince them that I could go to college, and they didn't want to recommend it if it was plain I couldn't do it. I must've done something right; I'm in college now.