Accepted Answer

That is a tough question to answer,
It requires a skilled professional to diagnose Autism, since there is no simple blood test (or similar) for diagnosis, and this is essentially a behavioural diagnosis.
As a screening tool...

• Play. Children with autism fail to engage in reciprocal play, where there is a back-and-forth between two people (as when rolling a ball) and functional play, where children play with toys such as dolls or cars in the way intended.

• Motor imitation. Children with autism copy others’ motor movements far less than other children.

• Joint attention. Unlike most children, children with autism don’t often use pointing or other techniques to direct another person’s attention.

Source

Here are a few more important screening markers in younger children...
The National Institute of Child Health and Human Development (NICHD) lists five behaviors that signal further evaluation is warranted:

• Does not babble or coo by 12 months
• Does not gesture (point, wave, grasp) by 12 months
• Does not say single words by 16 months
• Does not say two-word phrases on his or her own by 24 months
• Has any loss of any language or social skill at any age.

Source

All these kids will need to see a pediatrician skilled in neurodevelopment, or an allied specialist, who will be able to provide a diagnosis,
regards
Dr. Gupta

Expert:	MD doc
Pos. Feedback:	97.3 %
Accepts:
Answered:	11/5/2007

Doctor (MD)

Physician.