> the same sort of questionnaire that you'd find on the Internet.
Unfortunately the methods vary, and not all professionals care to (or even are trained to) conduct the more elaborate diagnoses. However, for differential diagnoses (ie, the most difficult to make - 'is this really ADHD, or just the way that Joe's depression/OCD/etc. happens to manifest?), there are some more elaborate diagnostic methods. I can't comment firsthand, but I know these tests can span multiple media (in-person interviews, written self-evaluations, third-party reports, IQ tests, fine motor skills tests, achievement tests, etc.). I had a friend who went through a battery of these tests before her final diagnosis.
Very few people actually bother, because it's very expensive (and often not covered by insurance). The only reason you would go through this is if you want to be really sure that you have ADHD, or if you're trying to identify whether cognitive treatment (ie, therapy) is appropriate or whether a hidden neurological condition, etc. is actually the problem.
However, even for a professional, it's almost impossible to make an accurate self-diagnosis (or even a diagnosis of one's own children, for example). It's just too easy to fall into the trap of confirmation bias - or worse, ignore signs that may be visible to others, even if obscured.
I do agree that it's unfortunate that not all professionals are properly equipped to make these diagnoses - this is especially the case for children - because it leads to an overdiagnosis among healthy children, while those with less obvious (but still debilitating) symptoms slip through the cracks without ever realizing they have a problem, since they are able to 'make do'.
> (Though perhaps the tester was also observing my behavior, and the like.)
I am almost certain that this was the case as well (at least to an extent)!