Today, it’s really not possible to diagnose an infant younger than 18 months with autism. If you are concerned, however, you can always monitor and track your child’s growth and development milestones.
There are some very useful checklists as well as growth and development tables available at http://www.cdc.gov/ncbddd/autism/ActEarly/ccp/downloadmaterials.html for children of all ages. These milestones do not apply to just autism.
You should look to see if your infant is not meeting any of these normal milestones. It is important to remember, however, that all children and infants develop at a different pace. Just because your infant does not reach a certain milestone on the normal cycle does not mean your child has autism or any other condition, it just might mean you should consult your physician.