My son has Aspergers so he is on the very mild end of the spectrum. He wasn't diagnosed until after age 4 when he started preschool and we could see him side by side compared to many other children his age. He didn't play with the other children, he always seemed a few steps behind. He was "shy" but to an extreme. He didn't answer questions for the teacher or volunteer for anything.
In hindsight, here are some signs we missed at home when he was younger. He loved Blues Clues and used to play this game at the bottom of our stairs where he would throw a ball up the steps, yell "Blue Scadoo You Can To" and then catch the ball as it came down the steps. He would do this for long periods of time over and over again and if someone else came into the area and touched the ball he would have a tantrum. When he would get excited about something he would jump up and flap his hands. Sometimes it appears like any other little kid getting excited about something, but my son would get up and do this even if he had been sitting in a chair or on the floor. Even now at age 8 if he's playing a board game and gets excited about something (or even if it's nothing too exciting) he'll suddenly jump out of his seat, go hopping half way across the room and then just come back and sit down. My son also does not like to get help with anything. Trying to teach him to catch a ball, write, shoot baskets, play video games, he doesn't want any instruction and throws a tantrum if he's not able to do it perfectly right from the start. He's better about this now, but when he was younger it was frustrating.
With Aspergers the flip side is that they are usually highly intelligent. My son memorized all of the names of the dinosaurs and the trucks in his "My Big Truck Book" and "My First Dinosaur Book" at age 2 1/2. You could flip through to any page and point to the truck or dinosaur and he could name it. My son was reading at age 4 just from my husband and I reading to him. He had not gone to school and we had not specifically sat down and taught him to read, he just knew what to do from having books read to him.
My son also gets "obsessed" with certain topics or TV shows. When he was younger it was the Backyardigans or dinosaurs. Now that he's older it's baseball (my husband is thrilled :-)) and the gameshow "Lingo". He DVR's it and watches them back over and over again. He also plays the game on my iPod Touch.
Good luck,
K.