Various. Psychologists, pediatricians, family docs, psychiatrists and others may be trained to test for adhd, but not all offer this testing. Testing usually involves interviews & checklists with child, parent(s) and teacher(s). Best to start by asking your pediatrician or family doc; if they don't do the testing themselves, they can refer you to the right qualified expert in your local area.
Thoroughly . Many conditions "look like" adhd. Evaluation includes questionnaires, extensive history, physical & neurologic exam, & neurodevelopmental assessment. This will determine presence of adhd, & provide target outcomes for treatment. See http://pediatrics.Aappublications.Org/content/early/2011/10/14/peds.2011-2654.Full.Pdf. This is the standard from the aap.
Criteria & surveys. It usually starts with filling out forms (like vanderbilts or connors) to determine if you have the signs and symptoms of add (inattention)/adhd (hyperactivity and impulsivity). Then there are criteria such as age the symptoms started (before 7), if it affects you in more than one enviroment (work & home), that it causes a significant problem & that it's not explained by another medical problem.