Because there is no fool-proof or medical test for autism, any child suspected of the disorder has to undergo vigorous screening from all sides - genetic, physical, psychological, etc. All that can really be used is whether the child is observed showing signs of communication or behavioral problems as well as developmental abnormalities. The trouble with diagnosing autism is that some of the symptoms are shared with other diseases and disorders, such as hearing loss, a behavior disorder, another pervasive developmental disorder, Rett syndrome, or a metabolic disorder - they may even just be odd or eccentric without having autism. Some of the tests done during screening will be to eliminate these other possibilities.

The DSM-IV is the main diagnostic tool, and diagnosis is determined on observation of behavior. A diagnosis of autism requires at least six symptoms provided in the DSM-IV, which accounts for the vast spectrum within an autism diagnosis. At least two of these symptoms should imply impairment within interaction in social situations, such as the following: an inability to manifest or interpret nonverbal cues, inability to develop friendships at the level expected in the child's age group, inability to initiate social interaction, and trouble with social reciprocity. At least one of the symptoms should imply impairment in communication skills, such as the following: difficulty with or lack of any language skills without the usual attempts by children to communicate in other ways like gestures, difficulty in starting or sustaining conversations if the child does have adequate language skills, and a lack of imagination and impulsiveness in play. And at least one of the symptoms should include limited or repetitive behavior, such as the following: focus on one or a few activities at the expense of other activities, inflexible need for routine, repetitive movements or speech patterns, or a focus on the parts of an object rather than the whole. These cover the general basis of the definition of autism within other pervasive developmental disorders. The onset of autism has to occur before three years old in order to be considered.

Other diagnostic tools are available to doctors. There is the Autism Diagnostic Interview-Revised (ADI-R), which is a loosely structured interview for the parents. The Autism Diagnostic Observation Schedule (ADOS) is implemented through direct observation of the child. In most clinic settings, the Childhood Autism Rating Scale is used in order to arrange the possible severity on an autism spectrum through observation of the child.

Diagnosis usually occurs when a child is around three years old in order to account for normal developmental delays. The diagnosis can be posited as early as 14 months, but it is much more likely to be a credible diagnosis after some time has gone by. In addition, the diagnosis is still early enough to allow parents to prepare for the different methods of raising an autistic child.

It is believed that the increase of autism diagnoses in the country is also due to improved and refined diagnostic practices - in other words, that they are able to catch autism better than before rather than confuse it for a different disorder or disease. Over diagnosis can be a problem since drug treatments give some doctors incentives to provide a diagnosis of autism; yet under diagnosis is also an issue because the cost and time of some of these test inhibit some parents from being able to pay for the tests necessary for a diagnosis.