There are a few tell tale sign of autism, they're called stims. A few are avoiding eye contact, not very much emotion ( smiles, laughs), and a BIG one is being entertained by simple things like if he'd rather watch the ceiling fan go round than an interactive children's show. Hmm, another I've heard of very often is playing with oys but not in the correct way, example: a toy truck, instead of making it drive around and making vroom noises, an autistic child would turn it upside down and spin the wheels and just do that for fun.
Hope i was a help.