What is the best approach to get my son checked for ADHD?

Start w/ pediatrics. Consult your pediatrician first for screening and to rule out other problems such as sleep apnea, hearing loss, anemia, iron deficiency, thyroid problems. The best questionnaire is the achenbach child behavior check list (cbcl). If needed, a developmental pediatrician or child neurologist or psychiatrist may be consulted for diagnosis, treatment or follow up.
Evaluation. You can bring your son to a board-certified child psychiatrist, or a behavioral neurologist for evaluation. They can review his history, your observations & possibly those of his teachers. Also they can speak with him and do some testing which may substantiate or rule out adhd. They can make treatment recommendations once this thorough evaluation is completed.