It does depend on what sort of testing you are having done as well. With DS5 we had some concerns about ADHD, but knew he was clearly gifted. Because some aspects of giftedness can appear superficially similar to ADHD, having a tester familiar with both presentations was important to us.
While there's a fair amount of objective testing that can be done, a lot of the diagnosis process involves personal judgments as well, especially to put all the disparate testing information together, especially if you are concerned about dual diagnoses (i.e. having both autism spectrum and giftedness). In addition, the presence of giftedness can have an effect on recommended interventions. Dr Amend has a fairly accessible book on the subject (Misdiagnosis and Dual Diagnosis of Gifted Children and Adults) if you want to get deeper into the topic.
WRT occupational therapists, and I recognize that my view is likely a minority viewpoint on the board and probably a function of small sampling, I find that OTs are really good at seeing stuff that they can treat regardless of its presence or the efficacy of the treatment.
