Actually, there are a whole bunch of distinctions within ADHD.

First, there's the common ADD versus ADHD. But a Dr. Amen makes multiple distinctions beyond that. He's got six types. In terms of satisfying your standards for ADHD being real, he classifies based on SPECT imaging and behavior, claiming correlation, though I can't remember the specifics now.

I'd say that the brain can screw up in lots of ways, and lots of those ways get called ADHD, but I would expect fuzzy and sometimes contradictory classifications because most are based on behavior, and not on detailed white box measurement of what is going on in the brain. Different ways of sub optimal function will get boxed together based on some behavior, but then unboxed with finer parsing of the behavior.

http://www.amenclinics.com/conditions/adhd-add

I was diagnosed, or at least given the option of trying medication once. Adderall didn't do anything for me. But I read up a bunch, as we are wont to do around here. One theory not generally advertised is that ADHD is about having too low a basal level of activity in the prefrontal lobe, so that you're too easily distracted by acute stimuli.