Your first step should be to consult with your own pediatrician. It's important to note that most pediatricians have little experience with autism, and there is no simple medical test to check for the disorder. Because of this, you may hear from your pediatrician that you're worrying too much, that all kids develop at a different rate, and that he's doing fine. Your pediatrician may be absolutely correct, but it's always possible that he or she is mistaken.
If you still have concerns, ask him/her for a referral to a local clinic, hospital program or developmental pediatrician or neurologist who has significant experience in diagnosing developmental disorders. Also be sure to find out whether there is an early intervention program in your state that offers a multi-disciplinary evaluation free of charge.