You usually shouldn't have to wait too long before being able to tell if an embryo has formed inside the egg. From just the fourth day, you can use an egg candlier or ovascope high intensity egg candler to see inside the egg. If you see a spot in the center, that's a sign that the embryo has formed and fertilization has occurred. However, with a darker shelled egg, it might be a bit trickier to see inside it at such an early stage, even with an ovascope high intensity candler, so you might have to wait longer. In any case, if you see the spot in the center with veins around it, the embryo has formed. If you see a ring of blood, detect a strange smell, or can't seem to see the spot in the middle, it's likely that your egg has not formed an embryo and should be removed.