At Three Years
When your child is three, pay attention to how he is handling more complex communication or situations. By now, basic understanding of grammar—such as the use of "he" versus "she"—should come naturally, and your child ought to be able to express emotion either verbally or using facial expressions. It's also time to introduce your little one to more complicated toys, such as puzzles with four or five pieces and objects with levers and buttons. If he struggles with this type of communication and play, that's worrisome. So too is your child's inability to relate to others. Showing concern for a crying friend or being wary of new people is normal at this age. If your child seems to demonstrate a lack of empathy or a disregard for other people, this may be a red flag: Children with autism often struggle to adapt to social settings.
At Four Years
By now telling stories and singing songs from memory should be a cinch for your child, and following instructions with more than one step should come naturally. Cognitive abilities usually include being able to turn pages of a book, open doors, catch a ball (some of the time), draw shapes, and count. Your child should also be able to communicate with his or her peers and jump at the chance to do so. At this age, it's important not to mistake shyness for your child's inability to handle his or herself in social situations. But if you notice your child struggling to relate to familiar children by sharing toys, playing make-believe games, or carrying on simple conversations, talk to your pediatrician.
To learn more about picking up signs of autism, visit the Centers for Disease Control and Prevention's website.