There's no one gene, but aside from very specific things, there's hardly one gene for any condition, but ADHD is highly heritable, and we already know several strong candidates. DRD4-R7, COMT Val158Met, TPH2, a couple others have been identified recently. Also, PET scans and in some cases high-density EEG can pick it up, the thing is, those are quite expensive and don't really even tell you that much, whereas psychological evaluation tells you much more qualitatively.
There is neither one or many genes or any objective biological test that is used to diagnose any psychiatric condition in the DSM.
"psychiatric diagnosis still relies exclusively on fallible subjective judgments rather than objective biological tests"
-Allen Francis, Professor and Chairman Emeritus of the Department of Psychiatry and Behavioral Sciences at Duke University School of Medicine, chair of the American Psychiatric Association task force overseeing the development and revision of the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV)
The mechanism of gene expression means the presence of certain genes may cause an individual to be more or less susceptible to certain stimuli.
So yes, the presence or absence of genes alone may not determine much, but together taking account to one's environment and upbringing, may have explanatory power.
It's similar to medical diagnoses where the presence of X genes mean someone is Y% more likely to exhibit Z condition.