I have 3 with ASD. With all of them, I knew that their development was not typical.
Eldest, I knew from around 6 months old. I didn’t know what we eventual diagnosis would be, but it was as clear as anything that she was not developing typically. She has severe ASD and was diagnosed at 2 1/2, having been ‘in the system’ from around 9 months old.
Middle child was more subtle, but by then I had done a lot more reading on typical development. She had a LOT of sensory issues, and by 10 months or so it was apparent she was not developing typically. She has Asperger’s (as was, her ex was a while ago!) and was formally diagnosed at 7, although it was clear long before that.
Youngest was the most typical in terms of development, but even so, it was plain by the time he was 14 months old. He has mild-moderate ASD, and was diagnosed by the time he was 3, having been in the system since about 16 months old.
Looking back, with all of them, it was not so much any one particular behaviour that flagged anything to me, but rather that their behaviour as a whole, and their reactions to everyday people and events, highlighted that they were not behaving/reacting as I would have expected them to.
OP, I’d say keep a watchful eye, take note of things that concern you, speak to your health visitor. But don’t forget to enjoy your baby - time really does fly by (mine are 17, 14 and 10 now!).