For a novice or beginner it's usually best to wait til day 10, you should really be able to see the embryo and veining. Clears will be apparent or should be. Day 5 or 7 if you know what your looking for. I don't throw anything away til day 14 if I have marked them with a question mark as a (hmm not sure egg) lol
I think it all depends on breed and color. I can usually see on day 3 or 4 on my silkies. I never cull till day 10 thou. I have set some dark brown eggs before and could tell till about day 10.