Comment by InSteady

This is being investigated and may eventually become part of the diagnosis (if you can afford the tests):

https://www.frontiersin.org/articles/10.3389/fpsyt.2023.1026...

>In conclusion, a series of biomarkers in the literature are promising as objective parameters to more accurately diagnose ADHD, especially in those with comorbidities that prevent the use of DSM-5. However, more research is needed to confirm the reliability of the biomarkers in larger cohort studies.

But yeah, generally there are a lot of conditions where you go report symptoms to your doctor or perhaps a specialist and they prescribe a treatment based on that alone. Testing is mostly used to rule out the really nasty possibilities or figure out what's actually going on when first-line treatments don't work.