Do some research on the behaviors and traits of children with autism spectrum disorders, and complete a little bit of self-diagnosis. Does your child appear very sensitive to loud noises or strange smells? Is his speech delayed? Does he appear extremely antisocial around his peers? Are his interests unusual for a child his age? If so, it's time to seek a second opinion.
Visit your pediatrician and discuss your concerns. Your pediatrician should be able to give you her opinion as well as give you some suggestions on places you can call or go to within your community for additional assistance.
Contact the Autism Society of America, and see if your community has a local branch. If not, the Autism Society of America should be able to give you names and numbers of local places to search. There are other organizations that deal with autism as well who may have slightly different names. They often provide testing, and if they don't, they should be able to lead you to people who do.
Call the special education department at your local public school. Many public schools provide testing for children regardless of whether your child is in the system or not. If they do not provide testing for homeschoolers, they should be able to provide you with names and numbers of additional resources that will be able to help you.
Check community resources or calenders for local autism support groups. The people who belong to these support groups deal with autism every day. They should be filled with ideas, as well as names and numbers of places within your community that have helped them.