No single answer. Autism is diagnosed on the basis of observed and reported behaviors. There are structured tests that systematize this process, but it is a clinical diagnosis. The clinician is looking for persistent problems in social interaction, language, and unusual behaviors. Experienced clinicians often make the diagnosis within seconds; inexperienced ones can use the formal criteria and misdiagnose.
The American Academy. Of pediatrics first recommended in 2002 that pediatricians perform developmental surveillance using standardized screening tests ( asq or peds) at all well-child visits & autism-specific screens ( m-chat or pddst) at 18 & 24 mos. Screens are being developed for earlier ages, since we now know that changes on brain mri's & diminished eye-to-face gaze can begin ~ 6 mos. See first signs.Org.
Therapist screening. Screening is done by psychologists and psychiatrists with a variety of behavior rating scales.