How might an HG+ adult go about finding a therapist when most of the issues they need therapy for probably cannot be understood by a person of high-average intelligence (which I'm assuming most therapists are...of)?

Have you had experience with therapists before? If so, what about the therapists you've seen did you like, and what didn't you like?

Generally - and this goes for everyone, actually - you have to kiss a lot of frogs. Most client/therapist pairs aren't going to work. Whether a therapeutic relationship will click or not is very individual and idiosyncratic, and you can't predict whether it's going to work or not by the therapist's credentials, their stated psychological school of thought, or much else about them on paper.

They probably won't be able to understand/remember everything about you; there are places you'll have to go on your own. Again, this goes for everyone. No one can fully see into another person's mind. What matters is whether the fit is good enough - whether you feel like they get you enough for you to be getting something out of it. And this may change as the therapeutic relationship goes on - you may eventually decide that you've gotten all you can get out of working with a specific therapist and you'd rather move on.

There is no perfect fit. But there probably is a fit that's good enough. It's just a lot of trial and error to find that, unfortunately.

Read resumes and then ask.

My wife is a therapist, HG+, with many similar clients. A therapist capable of working on hg clients should be able to articulate what makes those clients different and if they have experience with that type of client.

At a minimum they should be able to reference their reading history on the subject. We all know the buzzwords, they should be familiar with some of them.