Scientists at Emory University measured infants' ability to respond to social cues while watching videos.
They are hoping results could change the way autism is diagnosed and treated.
Babies at both high- and low-risk of autism were tested as part of the trial, including Lucy Keane, whose six-year-old brother Patrick has Asperger syndrome.
People who have family members with autism are 20 times more likely to develop the condition themselves.
Director of the Marcus Autism Centre Dr Warren Jones said the trial was aimed at developing a new screening procedure that allows doctors to hunt for signs of autism before symptoms become apparent.
"What this study really shows us is that is possible to identify robust signs of social disability within the early signs of life and these are the earliest ever identified signs of autism," he said.
"We tracked the way babies were looking at out the world from the second month of life all the way out into 24 months of life and even at 36 months of life when we confirmed the final outcome diagnosis for the children.
"Our hope is that by developing these kinds of tools which are objective measurememnets of a baby's behaviour, we can speed that process of early detection and get more children the kind of intervention services that would really help in the long term."
