Me again (i keep thinking of extra things!!) one clue to be found in 3 to 4 yrs is the way they play. In a former life, i was a speech language th (not specialised in autism but all SLTs are trained to analyse kids' play as symbolic (pretend) play closely aligns with language development.
For the past 25yrs though, i have been a childminder and observed hours and hours of NT kids playing.
NT kids aged 3 - if they have a toy with little figures and accessories (dolls house type - playmobil or anything like that)
And you sit back and watch - not show them what to do - they will start making a little play 'scene' by themselves. Often accompanied by chatter even if they are on their own. So they will pretend a little figure is hurt, or tired and put it to bed. Or a boisterous boy might pretend to kill some of his figures (this is normal in boys - car play and gun type play) But, whatever happens, you can observe the child pretending, making a little story totally from imagination.
ND kids either have no interest in this kind of toy or
Will start to organise it (tidying the furniture into the rooms)
Post things through the doors
You might see things that look pretend - making a figure jump off the roof, or be run over by a car, but it will end there. That will be it - more characters jump off the roof but it does not extend to anyone getting hurt, any doctor coming or any extension of the play idea.
Copying ideas they know from tv/movies - you might then hear sterotyped copied language from films.
Playing the same way over again.
If you have NT kids over to play and set them up with a toy like this - you will see and hear a difference.
Dress up and role play games (popular at 4yrs ish) are not favoured by autistic girls. NT girls generally love dress up and pretending to be the shopkeeper/nurse/mummy/princess or whatever and, by 4, they can create very long imaginative scenes with friends and no adult suggestion. You will see them embody the role of the character they are in - changing their voices and mannerisms. Pretend play is huge at 4yrs old.
My eldest dd avoided it by sticking with arts and crafts.
Youngest dd did play with dolls but it stayed simple - pushing dolly in a pram. Putting her to bed. Changing her clothes.
Neither embraced dress up role play.