How do people get tested for add and adhd?

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.