I spotted signs things werent' quite right from the very beginning. My son wouldn't make much eye contact. Everyone always told me newborns would gaze at you or when you breastfed them they would look at you, but my son never did this. He did look at you, but not nearly as much as a 'normal' baby. He did many things on time - smiling. laughing, babbling, crawling, walking, playing peekaboo, but he didn't do some big things like point or ever say mama/dada. He couldn't gesture (still can't very well, although he has just started pointing a month ago and he's two!) and doesn't wave even to this day. He also didn't really respond to his name until about 1.5. I had a feeling very early on; as wonderful and perfect as my son was to me, I knew there was something about him that was very different to other babies and toddlers.
Now he's older he has grown out of quite a few of the big red flags, but some major ones remain. He is so sociable now (when he knows you, he ignores strangers) and he points, tries to communicate, makes more eye contact, understands basically all simple instructions and can point to things on command (he seems to have a huge vocabulary for shapes, animals, numbers, objects, people etc. as he can point to them all when asked, but he doesn't say any yet). Musically he's very bright and has been able to hum nursery rhymes pitch-perfectly since he was 18 months or so (now he can hum about ten if you ask him), but my husband is a musician and my son is always surrounded by music, so I'm not surprised. Having said all that, he still doesn't try to talk properly (though he can copy a few sounds and says words with his mouth closed - so 'circle' (his favourite thing) becomes 'hm-hm'), still gets overwhelmed when lots of people talk to him or in crowded rooms, he doesn't interact properly with other children his age and will completely ignore strangers even if they talk directly to him. There are also days he isn't very responsive with even us.