In order to have fertilized eggs you first need to have a rooster in with your hens. Very important if you want fertilized eggs. Your hens will lay eggs without having a rooster with them.
The oldest and easiest way to tell if an egg is fertilized is called candling the egg. It is holding the egg up to a very bright small flashlight.
You don't want to candle the eggs too often as it may interfere with development.