From World Bird Sanctuary

How do you tell if an egg is fertile? You look for the veins!

To see the veins, we do something called “candling.” It’s when you hold a bright light up to the egg in a dark room to see through the shell. You can get special egg lights, but a cell phone light usually works just fine. Veins usually start becoming visible as early as 3-5 days from laying, but the best time to check is usually closer to the 7 day mark. Fertile eggs will continue to darken as the embryo grows, and eventually will be too dark to really see anything as the embryo takes up more and more space in the shell.

You can see the difference between the infertile and fertile Great Horned Owl eggs pretty easily in these pictures.

Infertile

Fertile