The National Institutes Of Mental Health provides some helpful insights into understanding ADHD:

If ADHD is suspected, the diagnosis should be made by a professional with training in ADHD. This includes child psychiatrists, psychologists, developmental/behavioral pediatricians, behavioral neurologists, and clinical social workers. After ruling out other possible reasons for the child’s behavior, the specialist checks the child’s school and medical records and talks to teachers and parents who have filled out a behavior rating scale for the child. A diagnosis is made only after all this information has been considered

More information on diagnosis

A complete overview of Attention Deficit Hyperactivity Disorder can be downloaded in pdf format and printed from NIMH:

http://www.nimh.nih.gov/publicat/NIMHadhdpub.pdf