How many conditions are truly dichotomous? Now that (I hope) most clinicians are getting away from the notion of the "paradoxical effect" of stimulants, what evidence is there that ADHD represents a distinct difference from "normal" other than in degree of difficulty achieving appropriate and necessary levels of arousal?
Compare the behavior patterns of ADHD with those of inadequate sleep: poor focus, inattention, forgetfulness, impulsivity, distractibility, emotional lability, poor decision-making, need for stimulation, poor self-regulation--and all of it improved by stimulant medication.
In other words, ADHD can be artificially induced by lack of sleep. What's more, in years of giving psychological and neuropsychological tests, I never saw one that was particularly accurate or reliable in making the call. I learned to rely on interview, observation, and history to determine which individuals were far enough from average to justify a trial on medication.