Autism is usually diagnosed in childhood, when a parent raises a concern about their child with their GP or learning specialist.

If autism is suspected, the child may be assessed at a child development centre to identify his or her specific needs. They can usually diagnose autism in children between the ages of two and three. The diagnosis is then based on observing the communication, behaviour and development of the child. Professionals will involve you in the assessment to find out about your child's development. There is no medical test (eg. blood test or brain scan) for ASDs.

A milder form of ASD, such as Asperger's Syndrome, is often not noticed until the child starts school, because many aspects of their development are normal. At school their poor social skills are more noticeable and they may show challenging behaviour.