Assuming that the female HAS been mated......
If you carefully hold the egg up to a light (flashlight is good to use... It doesn't emit much heat, which can kill the developing chick. Look quick as not to allow the egg to over heat) If the egg is fertilized, there will be a dark (opaque) "small blob". An unfertilized one will have a ring, or halo.... But nothing dark. Irf you don't see a "little blob"--- wait a couple days, and check again. It can be very tiny to start, and hard to see.
If it is fertile.... As time goes by you can actually see the chick in later stages, as it grows.
If you carefully hold the egg up to a light (flashlight is good to use... It doesn't emit much heat, which can kill the developing chick. Look quick as not to allow the egg to over heat) If the egg is fertilized, there will be a dark (opaque) "small blob". An unfertilized one will have a ring, or halo.... But nothing dark. Irf you don't see a "little blob"--- wait a couple days, and check again. It can be very tiny to start, and hard to see.
If it is fertile.... As time goes by you can actually see the chick in later stages, as it grows.