Hmm, what's her sleep like? What toys does she like to play with? What us her eye contact like? Is she meeting all milestones as she should? Does she seem to crave sensory input more than most of a similar age?
Does she show distressed behaviour above and beyond others her age?
Does she wave, and seem interested in interaction?
My son was diagnosed a few days after he turned 3. I knew from very early on that something was very different about him. He was jaundiced and had a jaundice check with a Consultant at about 6 weeks old. I remember saying to them, I'm not sure it's related to the jaundice but he isn't like any of the other babies. He wants feeding constantly. He finishes a bottle and screams 20 minutes later for another feed. If he is ever put down, he screams, he literally screams until he's going blue. Every car journey I take with him he screams from being put in the car until he's picked back up.
I would hold him until he slept, and I'd put him down 30+ minutes after, it didn't matter how carefully, it didn't matter if I transferred him in his sleepsuit, it didn't matter, he screamed bloody murder.
He has maybe slept the night through 4 times in his 3.5 years.
He was obsessed with lining toys up, and spinning things from very early on.
His language was delayed, and he actually lost words like mama, and dada. He stims verbally though, 123, ABC, the wheels on the bus, monkeys jumping on the bed, twinkle twinkle little star, head shoulders knees and toes. But he cannot ask for a drink, call my name, or answer his own name most of the time.
He sensory seeks with water to an extreme extent.
He gets overwhelmed, screams and throws himself on the floor, and hits, or bites.
He's picky with foods, and really struggles with prickly tags, and prefers to not wear clothes if possible.
His interactions with others are really quite severely limited. He has an 18 month old brother who has more words, and communication cueues than he has.
Not sure if this all helps at all! But these are the things that I noticed as the first things I realised that were different.