Believe me, you eventually figure out that your child is not like all the other children in quite varying ways, and when you hear of thousands of other kids who have those exact same differences, well, that makes for a diagnosis. Asperger's is definitely a syndrome. It has so much in common with autsim but with some drastic differences.
My son began picking out letters by his first birthday and was reading by 2.5. Reading ANYTHING, even the newspaper. He cracked the code in his head and read everything perfectly. You could read him a word like "tough" or "exponential" and that was it, he knew it for life. He understood even the most complex things very early.
He had two early obsessions. Before age 2 he was obsessed with what floor people lived on. He was obsessed with elevator buttons and floors. (We lived on the 6th floor of a high rise at the time.) By age 2, he had turned to cars. We could walk through a parking lot with him in his stroller and he would call out every single car's name and model. "Blue Foahd Tauwus" "white mewcedes benz" "gween jaguar" whatever. This may have been caused and assisted by the number of parking lots we were forced to spend hours in waiting for everyone else to finish their religious service or restaurant from which he was always hastily removed. We were thrown out of some of the nicest places. I shudder to recall my embarrassment.
And we were told by all sorts of "friends" and family members that "All he needs is a good swift kick in the pants." Finally I snapped, and hissed at one uncle "I could beat him until he was barely conscious and it would not make him change!" And indeed it would not have.
I only wish I would have known my son's diagnosis earlier. His talking at 9 months and being as verbal as a litigation attorney since then made it hard to fit autism to him. I only had him diagnosed at age 9. Sigh.