In retrospect, I should have known sooner.
1. At birth he was extremely alert. He was looking us in the eyes at the hospital. Nearly everyone commented on this, but we didn't think much of it at the time.
2. As a baby he slept very little, and was impossible to put down. He always seemed restless, but rarely sleepy.
3. I'd say when he was 2 we were pretty sure, because he had memorized every car make/model in parking lots as we went through and could "read" logos, like Target, Babies R Us and Pizza Hut. He also knew many types of dinosaurs by name and could classify them by herbivore/carnivore/omnivore, and also by Jurassic, Triassic or Cretaceous. By the time he was 4 we had preschool teachers confirming our suspicions. His preK 4 teacher and I were both in tears and hugging at our conference.
I think one reason we didn't catch on right away was that he didn't talk until he was 20 months, but he had a different sound for different objects, buzzing, whooshing and brumm brumm-type noises that we eventually realized formed patterns. He was also colicky and that threw us off a bit.
Our 2nd child was different, but also unique from other children we came across. He was very calm even though he also refused to nap. He liked to watch other kids.
By the time he was 3 he was solving multi-step math problems in his head and understood the concept of percentages. He was always easier to manage in play groups as he's a little more naturally social than his brother. The only way it's really obvious he's different from most kids is if you start talking math or science.
Have fun with your DD. I can't say that I enjoyed the toddler years, because they were so much work and as a SAHM I often felt isolated, but now that my kiddos are older I really miss those days.
