I didn't notice anything unusual about my sons (now age 7) until their second birthday. They were given 24 piece puzzles by my MIL (who, of course raise DH, so had some experience with this). I was ready to put them into storage until the boys were older, but turns out to be an appropriate difficulty for them. Then I did some research, and realized other things they did were also unusual. For some reason it hadn't occurred to me that a young toddler's first words usually aren't identifying letter of the alphabet. What stood out was that they were very very slow with gross motor and didn't speak many 'real' words at the time. Thus, we worried they might be slow.
One of the difficulties in understanding that my sons were not, in fact delayed, was the fact that they had very poor gross motor skills (still do) and followed the family tradition (from both parents) of talking late, and then suddenly becoming intelligible when they were speaking sentences - except letters, of course, and a handful of other words.
Looking back, I remember a comment or two from the nurses in the regular nursery (after NICU for prematurity) explaining to me that their eyes tracked better and they were more alert because they had 'more experience' looking around than children who spend the last two months in utero. That comment was 3-4 weeks before their due dates, and she was comparing them to other 4 week old babies so her reasoning for her comparison didn't make sense, but I didn't question it at the time, or think much about it because I didn't know there were any differences and I was a sleep deprived new parent. I remember repeating it to my grandmother when she commented on it.
My youngest is now 14 months old. She is entirely different from her brothers. She doesn't get overstimulated, but understimulated, which drives me batty. She has been physically very strong and advanced with gross motor stuff and also with her receptive language, loves wooden puzzles, knows at least some shapes and colors and a few letters, and has a terrific memory. One thing that is the same, is that she is somewhere on the slow side of things with expressive language and does not seem to want to follow a typical pattern of early speech (her brothers went from incomprehensible to an enormous vocabulary between 25 and 26 months old). She rarely tries to say the same thing more than twice, usually once, and if I don't get it, too bad. I was kinda hoping she would break family tradition there. Nope. I don't know where she will be, or is, on the gifted spectrum, but I suspect she is at least quite bright, but probably not PG. Just a guess.