When my firstborn was young I watched him carefully for signs of giftedness. I had some issues growing up as a gifted child, and frankly I was hoping that he'd end up "merely" bright -- but failing that, I wanted to be prepared to support him in any way he needed. So I was relieved when he showed no signs of early verbal ability or other language-related precocity, in fact he had some language delays.
But as he headed into kindergarten his interest in numbers was obvious, so on a whim I got him some Life of Fred books to read together. (I still remember joking about "putting underpants on your head before putting socks on your shoes is not commutative!") He absorbed the concepts quickly and easily and shocked me by the things he could do in the abstract (multidigit subtraction with composition and decomposition, or feeling his way through an understanding of negatives and infinities.) Around the same time, I started reading The Hobbit aloud to him and he not only showed engaged listening, but each day we picked the book up again he could clearly and concisely summarize the previous day's passage. Finally, that was also about the time that his baby sister (age 4) revealed that she could not only read, but fluently enough to read aloud multi-syllable words with lovely inflection. O_O So...I guess I knew when my first was about 6 and my second was about 4.
In hindsight there might have been signs for both of them earlier. I don't think it really matters though. I would hope it wouldn't have changed how I raised them. I still believe that early identification can have a detrimental effect if it causes parents to focus too much on academic acceleration and not enough on just exploring the world and using the senses and finding their place in society.
NB, my kids aren't tested and I have no way of knowing if they're DYS level or just MG or somewhere in between.
