Consult peditrician. speak with your pediatrician and have your doctor do a screening review. If necessary, get a referral to a psychologist who can make the diagnosis.
Developmental eval. If you are concerned your child is autistic to any "degree, " you should seek professional help through your pediatrician. Also see https://www.Firstsigns.Org/ for help. Autism speaks also provides good resources for concerned parents.
Ask. If you have concerns, no matter how old your child is, ask your pediatrician, who is capable of advising.
Slightly autistic. Some children if not stimulated enough look like autistic. If the child is aloof, not interacting with other people, loves to play alone, has a ritualistic behaviour and is thrown off by small changes to their environment, think about autism. Sometimes if you engage them into conversations, activities, encourage them for doing different activities they will come out of their shell.