You can literally see ADHD in a brain scan, especially if you contrast it with after someone has taken the proper medication for it. This isn't some sort of witchcraft, it's a physical thing that's measurable.
That's not how you get diagnosed however, nor do I know if you would receive a diagnosis of adhd when a brain scan shows behavior similar to that of ADHD but you show no symptoms and aren't seeking a diagnosis. If you go in for an ADHD evaluation you typically get a psychological assessment, potentially do some tests which aren't always conclusive and your symptoms will be evaluated. In that sense, it's not really measurable in the same way that we can test for a specific disease, infection or virus.
It's not a strict relationship as far as I understand though. As in, frontal lobe issues are visible on many brain scans, but those brain scans are not specific enough to map onto ADHD diagnosis. So we don't have a good diagnostic test right now even though we have many population-level confirmed differences that we can run tests and statistics on.
