At day 7, you should be able to see veins. But if your light isn't bright enough, maybe you won't. If you see a small black dot, like, 3-5mm in size, that is a tiny embryo developing just like it should. When you see a big shadow that takes up a lot of space in the egg, that is the yolk. If you can't see a black dot, it most likely isn't fertile. Word of warning though: If you're new to candling and you're not sure, leave them in the bator unless they actually smell bad! Just about every hatch I've done, there's an egg I'm certain isn't developing, then it goes ahead and hatches! I actually find it funny to mark the presumed duff ones and then wait for them to hatch out...