I just candled my Day 5 chicken eggs and one of them has an embryo and then a dark ring around the perimeter of the vasculature. I can't tell if it's actually vasculature or a blood ring. How do I tell the difference without the egg exploding eventually?! The other eggs don't have it.
It's hard to see, but if you kind of unfocus you can see the shadow of the embryo up near the top and the ring around the bottom of the egg

It's hard to see, but if you kind of unfocus you can see the shadow of the embryo up near the top and the ring around the bottom of the egg