Diagnosis is based on behavior, not cause or mechanism.Autism is defined in the DSM-IV-TR as exhibiting at least six symptoms total, including at least two symptoms of qualitative impairment in social interaction, at least one symptom of qualitative impairment in communication, and at least one symptom of restricted and repetitive behavior. Sample symptoms include lack of social or emotional reciprocity, stereotyped and repetitive use of language or idiosyncratic language, and persistent preoccupation with parts of objects. Onset must be prior to age three years, with delays or abnormal functioning in either social interaction, language as used in social communication, or symbolic or imaginative play. The disturbance must not be better accounted for by Rett syndrome or childhood disintegrative disorder. ICD-10 uses essentially the same definition.

Diagnosis basically involves first evalution wether the child shows certain early symtoms. Early symtopms vary from one child to another. Once the early signs are known then futher diagnosis on the patient is done.Diagnosis can be done using several diagnostic tools such as the Autism diagnostic interview-revised (ADI-R), its a semistructured parent view and the Autism diagnostic observation schedule (ADOS) uses observation and interaction with the child. The childhood autism rating scale (CARS) is widely used in clinical environments to access severity of autism based on observation of child.