Nanny,
The only way to find out if one's child has ADHD is by having him/her evaluated by a medical or mental health professional. I always recommend that the child first be seen by his pediatrician to make sure there aren't physical reasons that are causing ADHD-like symptoms. For example, allergies and sleep disorders can cause symptoms that look a lot like ADHD.
If the pediatrician clears the child, then it's time for a formal evaluation. Though some pediatricians are capable of doing that, most do not have the time or expertise to offer a full, comprehensive evaluation. Therefore, it's usually best to refer to a child psychologist, psychiatrist or other mental health provider who is very experienced in the ADHD evaluation process.
You can read more about Diagnosing ADHD Children by reading about it here.