You have to crack them open. There's no way to know without opening the eggs.
On the yolk, where the white spot is, there will be a white "bulls-eye" circle instead of a solid white dot.
Search "bulls-eye" here on BYC and you'll find plenty of posts about it, including some with actual photos of same. I think if you check the "sticky" notes at the top of the Incubation and Hatching forum, there's one with those photos.