I had suspected ASD since DS was a year. Signs were:
Light-sensitivity. Would scream street down if sun got in his eyes whilst in the buggy. And found it impossible to sleep during the day unless we made his room as black as a cave. Couldn't sleep in any other setting.
In his own world. Would do same bits of repetitive play for hours (mostly focused on opening and closing doors on cars, dolls houses etc) and arranging things by colour.
Didn't respond to his name.
Non-verbal but also very poor receptive language. Didn't seem to understand what was being said to him.
Poor fine motor skills, couldn't hold a plastic cup to drink easily. Couldn't hold a crayon.
Sensory seeking - wanting to spin, swing, walk on unusual surfaces, squeeze himself in tight spaces, lie down on hard surfaces etc.
No interest in other children at all.
I think that's most of it. But all ASD kids present differently so someone else will probably be along in a minute and give you a totally different list of symptoms....