I went to a clinical psychologist - I didn't get a referral from a doctor, I just referred myself and paid for the consultations.
First, I completed the AQ Test. Then I had a two-hour interview going over my whole psychological history. In the next week my wife completed the AQ Test about me (and she scored me within 2 points of my own score, without knowing my own score or anything about the AQ Test). Then a week after the first interview, we had a second, one-hour interview with my wife and me together, discussing each of the diagnostic criteria in relation to my behaviour. And then, at the third session, he said that I had Asperger's.
He is a specialist in ASD who was recommended by (and had worked under) an acknowledged national and international expert on ASDs. So I reckon that between his expertise and his thoroughness he is pretty reliable.