First let me share the tools that I've found helpful, and then I'll get into answering the question above a bit more thoroughly.
I am a big believer in parenting instincts when it comes to something being not quite right. And over the years I've found a few websites that can be really helpful in helping parents sort out whether or not things are outside the range of normal, which is really quite a large range, although not so large that some of us don't find ourselves, or those we love quite outside of it.
The first website that I go to allows you to take a test called the ASQ, online. The ASQ, or the Ages and Stages Questionnaire, can be extremely helpful when a child is missing key milestones. I first took it for Patch when our pediatrician's office used it before each Well Child check. I quickly realized that it was a very valuable tool.
In my experience it has been amazingly accurate at raising red flags when and where they need to be raised. And the best thing about the ASQ Oregon site is that it tells you where there are delays and allows you to print out your results and take them to your doctor to say "look, I was worried so I filled this out and these are the results."
It helps me explain exactly what's going on in a way that is easy for my doctor to understand, and our pediatricians have always felt that that test is a solid indicator for sending the kids on to the correct testing with other therapists or specialists.
The second test is the MCHAT.
The ASQ can be given to babies. The MCHAT is for toddlers who are at least 16 months old (the ASQ can be given to toddlers too). When you take the MCHAT online, like the ASQ, the results will let you know if there are concerns and you can print the form and take them to your doctor.
Having red flags on the MCHAT doesn't mean your child has autism, but it is a good starting place to get answers. Maggie had a ton of red flags and she is on the spectrum. James had five (which alarmed his developmental pediatrician) and his therapist and doctors are extremely pleased with his social communication skills at this point and if his extreme articulation of every feeling he has is any indication I'd be rather surprised if he ends up being on the spectrum.
Those are my two favorite starting points, along with finding a good pediatrician, preferably one with experience in developmental delays.
It took us a long time to find a doctor that was good for Maggie. At first we just went to the local pediatric clinic. We quickly realized that this was too hit and miss. On the one hand we met the doctor who told me "I am a hundred percent certain that your daughter is autistic" there. She was amazing. But once she moved on I had a doctor ask "what's OT?" when I was explaining the therapies Maggie was in and another tell me that nothing could be wrong with Maggie (who'd been running a fever for a month and a half) because she was too energetic.
We realized that we needed to find a doctor that understood autism, and who could see past the fact that Maggie could go from sleeping twenty two hours a day with a fever at home and not drinking any liquids for two days, to bouncing off the walls of a waiting room, because she has that sort of endurance when she's super stimulated.
The other starting point we've used is directly contacting our state's Early Intervention program. With Patch, the doctor contacted them for us. With James and Tessie we were already in contact with Early On (the Michigan program) because we had an older child in the program, and we made the referral ourselves (Tessie was actually spotted by three different therapists who suggested that I make the referral because she was so unresponsive when she was 1-6 months old).
Early intervention can be huge. Honestly I would rather take the chance of being overly concerned and be told that everything is fine, then miss a problem and have to deal with more therapy later on.
In a few months James will be graduating from all forms of therapy. He's been doing PT since he was seven months old (he couldn't prop sit on his own at ten months). And I credit the hard work from his early interventionists with the fact that I'm going to have far less appointments in the future than I would have if we'd "waited to see" what was going to happen.
He's worked really hard to get where he is and it shows.
Hopefully you'll never have to use any of these resources, but if you do it's good to know where they are.
As to "how did you know?" That's trickier. I didn't for a long time. I thought that we had late talkers. Maggie made eye contact with me constantly. She didn't maintain eye contact, but she made it.
For us the symptoms were specific, but I seldom see her when I see those "what to look for" lists. She liked spicy foods and strong flavors. She didn't appear to feel pain, especially when she was really hurt, and her responses to pain were inappropriate. When she was struck by a kid swinging and thrown about ten feet, she jumped up, giggling. She was very cuddly, but she always wanted her skin covered, usually in a sleeper, because light touch hurt. She wanted to be held and squeezed tightly. She never stopped moving. She broke everything, usually without meaning to. She was incredibly strong and incredibly coordinated. She made high pitched squealing sounds. She guided my hand to get things she wanted, and to point to things (once she got older).
She never rocked or flapped. She was incredibly cheerful and happy most of the time. She pointed. She made noises to get my attention. She played with other children. And she always noticed if someone else was upset and would come over and try to cheer us up.
Perhaps the biggest red flat though, was that she "had no tether" as James' therapist calls the thing that keeps him near me when we're out.
When I'm out walking with James I rarely have to worry about him going far from me, because I know that he will stay right next to me if he's out of his stroller. Usually he'll say "Hold hands! Hold hands PLEASE!" if I'm not holding his hand.
It is amazing to me, and fantastic.
Maggie on the other hand, had no tether from the start. Once she was old enough to crawl she was crawling away without looking back. And once she could run? We were constantly chasing her down, making sure she didn't sprint into traffic. It was terrifying.
I'm not sure my answer to "how did you know" is all the helpful because the saying "if you know one person with autism, you know one person with autism" is so incredibly true.
And that is basically the guide to the tools that I use when I have questions and the answer to the question of how we knew, or really, all the things we missed for quite some time until her pediatrician pointed it out.
In Tessie's case, being able to use these tools to express what was going on is what got her sent to the neurologist that ultimately realized that we weren't looking at a developmental delay (at least on its own) but that there was an underlying medical problem for her missed milestones, leading him to find the central apnea that had her so exhausted she had no energy for anything else. And if the number of times her monitor have sounded telling us she isn't breathing are any indication, that may well have saved her life several times over.
If you have any questions I can try to answer them, and if I can't there are usually other people in the comment section with invaluable knowledge that can!
All of the photos in this post are of Maggie. I used them because one thing I commonly hear is that if a child is happy and looking at a camera they obviously aren't autistic. That has never been our experience. Maggie was a challenging toddler, yes, but she was always extremely happy. And in my experience many kids on the spectrum are pretty curious about electronic devices. Looking at my phone that I'm taking a picture of her with is very likely to happen, possibly because she's plotting how to swipe it to watch Bubble Guppies and look at pictures of mermaids.