One autistic DC - everything had to be "just so" and if it wasn't then their reaction would be hugely OTT, hours long "tantrums" (which I now know were meltdowns) where nothing could placate them, they were stubborn to the point of absolute rigidity, it was their way or no way. Food issues and a diet that consisted of just 3-4 items, any other foods caused massive upset and were flat out refused. Hugs and affection had to be on their terms, if they wanted a hug then it was wonderful bit if you tried to hug them outside of this it was like hugging a stick as they'd stay stiff and unresponsive. Lots of shutdowns where they'd reach the point of overwhelm so would simply lie down wherever they were - living room, bathroom, supermarket, street - close their eyes and go completely unresponsive. Refused to wear clothes most of the time, very sensory avoiding. Played with toys but not in usual ways, like to have the toys but not for them to be operated or used, the toys were for placing around the room/house and for carrying but for actually playing.
Other autistic DC - total opposite. Sensory seeking so into absolutely everything, utterly exhausting trying to keep them safe as they needed constant supervision. Always on the go, always needing hugs, always climbing, always jumping, would talk to anyone and would think nothing of wandering off with total strangers. Loved toys but again not to play with, just to have. Would tip the toy box out, scatter them around, and then fill it back up just to empty it again as the end goal was simply to empty it rather than to play with any of them.
Both are really socialable but not always in socially acceptable or age appropriate ways, they are both also very empathetic but again not in an appropriate way, it's like they feel everything and can't filter out what's worthy of the emotional attention and what isn't. Both make eye contact although those is variable depending on mood, how many distractions are around and who is speaking. Both like to share their interests, at length, whether you want them to or not, and both are very skilled at turned a completely unrelated discussion back around to subjects they do want to discuss.
A turning point for both was when they got to around 4-5yo and I started to realise that my friends with similarly aged DC were getting to sit down with a coffee at softplay or at the park and I was still having to follow my DC around to supervise them, manage their interactions with others, and help them manage their emotions. I then gradually came to realise that my DC weren't like their DC and that my DC were developing differently. Once I could see it, I couldn't unsee it and it snowballed from there until they were diagnosed.