It's hard to say exactly. Given that DH is PG and I have an uncle who is PG as well, and my brother & I were considered gifted in math. It didn't seen entirely likely to look for giftedness in my children.
The movement I really KNEW DS was gifted was when he was a bit older than 2 years old. He loved floor puzzles at that age. I had purchased a Thomas the Tank puzzle for him online and it had just arrived & unwrapped when I received a phone call. Fifteen minutes later I finish my phone call to turn around and he had completed it himself.
That and when he started reading at 3 without much help from us except that we filled out house with books and that I was working hard with his older sister who has reading challenges and at 7 was still struggling to read.
