This video shows the subtle signs of autism from one child as he ages and his tremendous growth from years of therapy.
Of course, I have not included his other behaviors of tantrums, lack of social engagement, the fear of strangers, covering of the ears and hyper-focus of behaviors. Other than two word basic needs sentences, he only spoke in movie quotes until the age of 5.
Those signs are obvious, however there are subtle signs that show themselves sometimes before the obvious behaviors present. The earlier a child is identified to need extra help the better the developmental outcome.