Maybe this is your first child, maybe it is not. Maybe there is just a little worry niggling at the back of your mind. This child is not quite like your other children, or the the other children in his/her playgroup. What are some early signs of autism?
Lack of eye contact. Most babies will lock eyes with you when they are nursing or taking a bottle. A child with autism may not. I did not realize that my son did not do this until, two years later, I had my daughter. She may eye contact in the delivery room! I was stunned. Babies look at their moms? I had no idea.
Babbling/language. A neurotypical baby will babble and coo at you and is delighted when you babble or coo back. A baby with autism may not do that. They may not develop language on time, or develop a few words and then lose them (in the case of my son). A baby who is not on the spectrum will make faces at you and laugh when you make faces back. They can imitate facial expressions, even when they are tiny, such as tongue thrusting.
Play. A child on the spectrum will not usually play with toys in a typical ways. He/she may love cars, but only likes to spin the wheels. When you try to enter his/her world and play a game of pretend, such as put the car in the garage, he/she may become very upset. Your child may love stacking blocks or putting cars or toys in long straight lines. Woe to the parent who disturbs this line!
Sleep. Your child may not sleep, or may sleep only a few hours at a time. The rest of the household becomes zombies, but your child is still going strong. For a long time, my son did not sleep unless we wrapped him up like a burrito and then wrapped our arms and legs around him. He would struggle for a bit, then realize that he liked it and eventually give in and fall asleep.
Eating. Your child may be an incredibly fussy eater. Certain smells and textures are physically repulsive to him/her. They may live on a diet of goldfish crackers, and chicken nuggets. For a long time, the only foods my son would eat were pancakes, french fries, spaghetti, and chicken nuggets. It took years of OT and and patience, but he now has a very well rounded diet. He loves vegetables and ethnic foods, the spicier the better. But for a long time his diet was very limited. If this is the case of your child, you may want to add a multivitamin to his/her diet. Also, my son hated eggs. He would turn up his cute button nose (literally!) at foods that had eggs baked into them, even birthday cakes.
This is only a very brief list, mostly of things that I noticed with my son. But if you are noticing lack of reciprocal play, little or no eye contact, a certain adherence to rituals, language delay or language loss, I would strongly advise you to talk to your child’s pediatrician. The doctor can give you references to specialists, early childhood education, and a plethora of other useful materials.