Just because someone has difficulty concentrating or cannot sit still does not mean that they have ADHD. The only way to determine this for sure is through a diagnostic assessment by an experienced clinician. This takes the form of an interview to explore the presence of ADHD symptoms and associated impairments, their evolution in time, the impacts of coping strategies and also to identify other conditions that could mimic or increase the intensity of ADHD clinical presentation.
The clinician, together with the individual (and with family members if needed), will gauge the extent of the impact on their functioning in order to decide whether treatment is needed and, if so, what kind. Seeking out associated problems and conditions is essential in order to establish an effective, personalized treatment plan.
A neuropsychological evaluation serves to better quantify and qualify the cognitive impairments in order to work on them specifically in therapy or to confirm or exclude other possible diagnoses. However, neuropsychological tests alone do not determine a diagnosis (there is no specific test for ADHD).
There are questionnaires which make it possible to quantify the intensity or the functional impact of the symptoms and thus to measure the degree of severity of ADHD. Certain questionnaires can be filled out by the individual and their family members, whereas others require a particular knowledge and are done by professionals.
It is important to go beyond the symptom to question its origin. Somebody can be inattentive, agitated or impulsive for many reasons without suffering from ADHD.
The Canadian ADHD Resource Alliance (CADDRA) has established guidelines on the diagnostic process and the treatment of ADHD. The contents are revised periodically and are available on line.