when I cracked them open, I look for the germinal disk. If it's just a white dot, it is infertile. If there is a pattern of concentric white and darker yellow circles (like a bullseye with the white as the center dot) then it is fertile. I was unable to get a picture due to the glare. There's a thread around here though.How do you know of an egg is fertile?
https://www.backyardchickens.com/t/16008/how-to-tell-a-fertile-vs-infertile-egg-pictures