Several validated tools exist for diagnosing autism, such as the Autism Diagnostic Observation Schedule ( ADOS) and the ADI-R, a semi-structured interview conducted with parents to gather information about certain aspects of their child's development, provided the adult consents and reliable informants are available. Both tests have proven validity, but they also have some limitations, such as their validity in females and certain cultural biases, and require specific training to administer them.

Self-administered scales, such as the Autism Spectrum Quotient (AQ) or the Empathy Quotient (EQ), can also be used, provided the individual has good insight and no serious difficulties in comprehension. A neuropsychological and intellectual functioning assessment can also be helpful to determine the neuropsychological profile, particularly regarding executive functions (planning, cognitive flexibility, etc.) and adaptation, or to detect possible comorbidities.

These tests provide the specialist with the information needed to determine a diagnosis. The choice of tests depends on clinical judgment, as not all are appropriate or have the same diagnostic sensitivity. A thorough clinical interview is crucial, and the professional should be specialized in autism but also have a solid background in neurodevelopment and mental health in general, in order to make a differential diagnosis.

Autism is a multifactorial condition, not strictly linked to genetics alone, but also to environmental variables. Genetic testing is recommended in situations where the autistic person has an intellectual disability or there is a suspicion of a specific syndrome.