I found this interesting article on signs of autism in newborns and infants (suggesting that autism is a life-long disorder, not the result of a post-birth exposure). Researchers found subtle deviations that parents may not pick up on starting as young as the baby's stay in the NICU and growing more and more noticeable as the baby gets older. Finally, parents start to observe differences after children are over a year, "By about 13 months, the development of children with autism started to diverge markedly from those without, according to the study."
Very interesting. Unfortunately, the things researchers notice in autistic infants that deviate from the norm are kind of specific and you'd have to have seen a ton of babies to notice them... meaning it's not very helpful for parents spotting the disorder in their own children. Still, it's progress!