This question comes up a lot, and it’s a peril of knowledge:
“I think their kid might be autistic, but I don’t know what to say to them.”
My answers:
“I usually start with ‘Hi’ or ‘Hello.'”
“Do what I do – don’t say anything. Ever. For any reason.”
“You must be desperate to be asking me.”
This is a hard question.
Because you might be right, and their parents just might not know.
It’s hard not to intervene. To share and highlight where their child might be different. Where they could use understanding, support, people coming alongside.
It is the most well-meaning thing.
I don’t have a good answer.
I just have my own story.
Of course I feel I can spot other people’s children with autism right away. It sticks out, and even if it doesn’t, it sticks out to me.
I don’t have answers, so I do what I’ve learned to do best.
Talk.
Just talk.
No one really cares if you’re the first to suggest “the A word.”
But they care if you care about the things they care about.
If you’ve got neurotypical kids, share your challenges.
Tell your stories. Ask for theirs.
The common ground is where the truth is sown.
You can bring the warmth.
The growth is on them.
The Life Autistic 
This is SUPER hard. I so far have just gone with the “never say anything” approach. I figure at some point a teacher or someone will mention it. People get offended about it–not just about autism, but any suggestion that a child is different or needing extra support. My son has autism but many people don’t know that because he “blends in.” Because of that, I often notice other similar children. They can easily fall through the cracks, and the hardest is when you KNOW the parents know their child is different but they refuse to see it– or worse, get angry at the child for not acting like all the other kids. I saw that firsthand recently and it was one of the most painful things to witness. I still don’t know how I could have helped the situation or responded better. I just tried to recognize how well this child WAS doing and how sweet he was. But gosh it wrecked me.