Testing for ADHD is not a simple matter of taking a quiz, as some websites might have you believe. You need a mental health professional to receive an ADHD diagnosis.
Diagnosing a child typically involves interviews with both the child and parent or parents. A mental health professional may also go through some play and tasks with the child. Parents and teachers may also have to respond to surveys that the mental health professional will then use as a reference.
Diagnosing an adult involves interviews with the patient and discussing things like patterns of behavior or thinking. An adult may also fill out surveys during the process.