Use a small flashlight and place it next to the egg. The light will bounce off the incubation medium and light up the egg.
Egg only a few hours after laying. Egg 1-2 weeks old, very pink.
If you don't see any pink, just a yellow egg after 2 weeks the egg is infertile. However I wouldn't just through it out. There have been rare occasions for babies to hatch from eggs that look completely infertile.
