I think in ADHD as with most other disorders there are a lot of confounding factors that make diagnosing and studying the disorder particularly challenging. The higher prevalence of ADHD in the US may have nothing to do with environmental or social issues, it could simply be that there is more money and/or interest to conduct the studies. Age, gender, genetics, cultural background, exposure to TV, family and social structure, diet, presence of other conditions, IQ, emotion intelligence,... the list goes on. No matter how great the study design was there will be limitations and you have to be careful how you apply the findings. Also, just because something was found to be effective in a controlled experiment, doesn't mean that it will be similarly effective when used in actual clinical practice.

To answer the question the original question at the very least there would need to be a several studies where a large diverse groups of "norma"l health subjects were assessed using some standardize cognition test(s) and then given some stimulant medication and reassessed. This study would likely not make it though the IRB process. It would be difficult to justify giving normal healthy individuals stimulant medication just to see what affect it may have on them.

As for the diagnositic process... at this point there is no single measure that has high specificity and sensativity to differenciate between those you have and those who don't have the disorder. The best we can do at this point is conduct a series of assessments, look at the combined results and use this information to make an educated guess.