I know my parents said they suspected something was wrong with me when I was about 3 or 4 years old because I kept running into walls. Like for real... I would be in a room and I would turn around to leave but instead of going out through the doorway, I would turn and run right smack into the door frame. It happened many, many times.
It got so bad that my parents took me to the doctor to see if maybe I needed glasses or if there was something wrong with my eyes. The doctor told my parents that my vision was perfect. My parents didn't know what to do. I just eventually grew out of it and didn't run into walls anymore (I'm still clumsy now though). I guess I just learned how to make a better exit after a while, lol.
My dad later told me, when I was much older, about the wall thing. He said he used to be concerned that I might be, as he would put it, "retarded." It was only when I went to school and got good grades and the teachers would tell my parents about how smart I was, that my father dismissed the idea. So, yeah, I would say that was a sign.
Other than that, I have an early memory of shortly before I could walk. I just remember for some reason my parents had put me in a pink rabbit onesy. I remember it vividly. The onesy was like a full body suit made out of the itchiest fabric imaginable. It was like being in a suit made out of Brillo pads and it was hot. I felt like I was being roasted alive. It was pure torture but my parents thought it was cute, I guess. They still have a picture of me in that suit that I hate with every fiber of my being.
Another early memory was shortly after I learned how to walk. My parents also mentioned that I used to stand up in my crib and jump or bounce up and down for hours on end. I apparently did this a lot and liked it. My parents said they couldn't get me stop and that I would just jump and jump until I tired myself out.
Later, I enjoyed jumping on the bed until I got into trouble for it and was banished from any jumping. Then I went on to spinning around the room until I got dizzy and fell to the ground. That used to be one of my favorite games for a long time... Rather than play with other children, I liked to spin and make myself dizzy. So, yeah...
I was never diagnosed but I would say that I was an odd child. I didn't learn about or suspect I had ASD until just a year or so ago. So maybe some signs were there early on but my parents and I just didn't know how to interpret them.