Autism takes so many forms, so many paths. (Perhaps you've heard the phrase "if you've met one person with autism... you've met ONE person with autism.")
Each family has a different answer to the question "When did you first notice that something was unusual about your child?" That question came up repeatedly as we went down the path of seeking a comprehensive diagnosis for Joy.
So here's my answer:
We had two big measuring sticks for Joy's development. The first was her older sister Rose, born 28 months earlier. The second was Joy's daycare buddy, a little guy just two days older. Our families did a daycare swap for the first two years of their lives, so she had an exact age-mate who was always there.
Physically, Joy did some things before her daycare buddy, such as rolling over and crawling (and maybe walking too, if I'm remembering right). Those were all things she did at an earlier age than her sister too.
On the other hand, her sister was a verbal whiz, with 75 words by 14 months. Joy wasn't doing that. She seemed to have some babble, and I kept trying to count words, but they just weren't coming. Plus I'd think I was hearing a word, write it down, and then it would disappear. Elvis sightings! but we didn't know it then. Her daycare buddy wasn't as verbally precocious as her sister, but he eventually started to pull ahead with words too.
Those weren't immediately definitive, though. After all, there's a wide range of normal, and we kept reminding ourselves that comparisons can be less than helpful.
The first unmistakable red flag was a certain CD-player toy that we brought out at the age of about 10 months. If you pushed the yellow button on top, the "CD" would spin, colored lights would flash, a song would play. Then there was also a little noise-making barrel to spin, and a couple of noise-making buttons to push.
All Joy wanted to do was spin that barrel. She had a toy in her crib with a similar barrel, which in that case was the correct trigger to light the lights and play the music. But the CD toy required the push of the yellow button.
We showed it to her daycare buddy, and he "got it" right away. Showed it to him once, he was making it go again and again. Joy, on the other hand, took two weeks to get the idea, full of demonstration and hand-over-hand.
Another memory that really sticks out for me was some time later, age 18 months maybe, when we were embarking on early childhood Birth-to-Three evaluation. I commented to the first caseworker that when Joy and her sister and her buddy were at our front window, and I'd be pointing out something like a squirrel or rabbit, Rose and daycare buddy would take an interested look and Joy generally would not. I remember a bit of self-deprecation as I said that, qualifying it with a "maybe I'm imagining things-- might not be important anyway."
I wasn't imagining things, and it's important.