There is no one test that can confirm/refute a diagnosis of ADHD.
As polarbear discusses, proper diagnosis requires several different tests, evaluation and input from parents and teachers.
Certain dips in processing speed are common with kids who have ADHD, but those dips don't necessarily mean the kid has ADHD, nor does not having the dip mean for certain they do not have ADHD. The diagnostic criteria for ADHD is a constellation of symptoms beyond a certain length of time and having been observed since early childhood (I believe the DSM-IV still says prior to age 7.)
