How can you determine if someone is autistic or mentally retarded?

They often co-exist: Pediatric (neuro)psychologists use age-appropriate standardized tests of cognitive & adaptive skills to determine Global Developmental Delay through age 4, then Intellectual Disability. A DSM-V Autistic Spectrum Disorder diagnosis requires qualitative impairments in social communication & reciprocity + stereotypies/narrow, restricted areas of interest; specific tests quantify the degree of ASD.