Quick I'm-not-aeh response :P
In our experience, as far as hard data it's really just the questionnaires for parents and teacher. However - big however - that data gets put in the context of clinical observation throughout a bunch of different kinds of tests (ability, achievement, memory, other more-LD specific stuff), and gets some additional substantiation from some of those results (e.g. working memory and sometimes processing speed tend to be lower in kids with ADHD - but also in kids with almost any LD). For my DD, our psych also recommended testing auditory processing, as DD had some flags, and ADP can look a lot like inattentive ADHD.
So it's a lot of diagnosis by elimination. Do they have the behaviours associated with ADHD, consistently in a variety of environments? Do test results and clinical observations show patterns consistent with ADHD, AND not suggest any other (or additional) causes?
There's huge overlap between ADHD, LDs, ASD, OCD etc, so all this is complicated by the fact that is more likely than not to be ADHD plus something else.
