How does one find a good psychiatrist/psychologist? My experiences have ranged from "very bad" to "OK at best" and I think finding someone a bit more invested into figuring things out might help a lot.
If you’re focused in particular on getting a diagnosis for something, I’d also recommend reading relevant parts of “Psychopathology and Mental Distress” which is a tomb of a book that aggregates both DSM and ICD diagnostic criteria, but also provides current meta surveys on relevant research, from multiple perspectives. It’s reasonably easy to read but definitely better after getting the basic thinking tools from a proper course on clinical psych.
I mean, it's the same question as "How do I assess whether my plumber or mechanic or electrician knows what they're doing." Sure, sometimes there's an easy, obvious difference, but you can't really know for sure whether or not your electrician is grounding your entire house to your hot-water pipes unless you have enough skill and expertise not to need one. After a point, working with any professional requires a weird blend of faith and of assessment you don't have the expertise to actually do.