You can't tell from the outside if an egg is fertile, if you crack the eggs open and look for the spot on the egg yolks for a few days in a row and they are fertile, you can figure that the next ones are. Once an egg has been incubated for about a week, you may be able to tell if a chick is developing in it if you candle it. Here is a good link from the Learning Center on Incubating and Hatching eggs... it has pictures of fertile eggs and what you would see candling if you incubate them https://www.backyardchickens.com/a/hatching-eggs-101
