I had no idea and was actually very resistant to the idea, because DD is so like me (so all her autistic traits seemed - indeed are - pretty normal to me). Our nanny picked it up first.
Earliest things that I now link to her autism (though I'll never be sure): very placid baby, very happy and easy to entertain. Great sleeper but terrible feeder (went to various BF support groups who were all a bit clueless how to help). Got there in the end though.
She was a very late walker (and even stander) - hated putting weight through her legs so bum-shuffled everywhere - but was never that motivated to move really. Could entertain herself with whatever was within reach (see comment on easily pleased/easy to entertain above!) I do remember her bum shuffling at speed around the floor of St Pancras station aged around 20 months. She was fast by then!
She was a good talker and started talking pretty early but looking back probably had a bigger weight towards nouns/labelling things. Didn't respond very much to her name. Loved to show us things (joint attention has never really been a problem), but didn't point unless asked ("where's the ....?") and didn't ask for things like toys/drinks/snacks (which was what prompted our nanny to suggest SLT), and together with the Physio support for her late walking that set us on the road to a more general review and eventually her Dx when she was 3.
Later on when she started nursery at 2.5 she presented as "shy" - she was much more comfortable with adults than the other children, and it took a while for her to start to play really with her peers rather than along side. Hid under the table when there was a fire drill. I think Nursery might have picked it up then if we hadn't already been on that path. Great sense of humour. I remember her first nursery teacher (age about 2.5) saying she was the first kid in nursery to start to tell proper "jokes" and always very giggly.
Now she's 7 and doing very well. Has friends, doing well at school etc. But she is in a small class and can get a bit overwhelmed with new things or being the centre of attention (e.g. won't let anybody other than family take her photo). So some things are a bit harder for her but she has a nice life overall.