Normally eggs will candle pink or yellow the first week or two. They should start to chalk in a that period of time with the outer shell becoming very opaque white, like someone colored it white with a white chalk. That is the sign I look for to feel I have a good chance of them being fertile - but that still is not a certainty. With 100's of eggs over the years, I have never been able to really see veins candling. I look for chalking and incubate anything until it spoils.