In my case, I was referred to a psychologist who specializes in Adult ADHD. She then did a very comprehensive test (took 2 sessions of 3 hours), which included an IQ test: Wechsler Adult Intelligence Scale–IV (people with ADHD have very skewed results with usually bad results in processing speed and working memory), some questionaires and discussion.

I then was referred to a psychiatrist to discuss treatment and to an ADHD coach.

I do know people who have gotten a very quick diagnosis with a 20 minutes questionnaire but I wanted to actually be sure of the diagnosis and not being diagnosed with ADHD just because I somehow self convinced myself that I had it by reading online.