DS6 knows because of the readiness grouping they do at school; it's impossible for him and the other kids to not notice, really. He's in a multi-age classroom, and he does math and "word study" (spelling, grammar, etc.) with the second graders. The other kids see this, and they talk about how smart he is. They speak kindly, fortunately. And they respond well to his bubbly enthusiasm for learning math and science and Spanish - he's a class leader in that sense, even though he's not especially sensitive to other kids' abilities or lack thereof. He just wants to learn cool things, and he assumes they do too!
He's still ahead of the second graders in math by a long shot, and again, it's not hard for his other classmates to notice and talk about it. He's known all his life that math is pretty special to him, and that he's better at it than most people. When it comes up in conversation, I sometimes balance it by talking about the strengths that some of his friends have -- that D. knows a lot about history, that S. can create amazing drawings, and so on. I also let him know what I and his dad are good at (and not so good at), for perspective. But I never put him down by saying, "Yeah, you're good at math, but you're not so good at [whatever]." His teachers don't, either.
Occasionally DS6 and I have talked about DNA and genetics. I use that opportunity to talk about abilities that are "in our genes," like the instincts and behaviors that other creatures are born with -- but that what we choose to do with them is entirely up to us.
