Early intervention in autism has been shown to improve outcomes dramatically. It is therefore important to recognize the signs of autism as early as possible. Research has confirmed that appropriate screening can identify children at risk for autism from as young as 12 months of age. Screening tests such as the Modified Checklist for Autism in Toddlers (M-CHAT) have therefore been created which can help detect abnormal behaviors that should lead to formal evaluation and testing by a trained pediatric care provider. This simple online screening tool can be completed in just a few minutes and can be found at https://www.m-chat.org, http://www.autismspeaks.org/what-autism/diagnosis/mchat and other websites. This screen should be completed by parents who are concerned that their toddler may be showing some signs of autism.
While there is a broad range of normal childhood development, and having a delay or abnormality in speech or social development does not mean that a child is autistic, nevertheless there are certain milestones that if not reached should act as red flags. Any child who exhibits any one of the following must proceed immediately to be evaluated by a trained pediatrician who is familiar with the early signs of autism:

No babbling by 12 months

No gesturing (pointing, waving, etc.) by 12 months

No single words by 16 months

No spontaneous 2-word sentences by 24 months

Any loss of language or social skills, at any age

There are some who advocate immediate evaluation for some earlier signs as well:

No big smiles or other warm, joyful expressions by six months

No back-and-forth sharing of sounds, smiles or other facial expressions by 9 months