If you like you can check the fertility of the eggs at about 7 days after full incubation has commenced. It is easy to do as displayed in the pictures opposite.

I have made my own holder from a scrap piece of wood. It is much cheaper than buying one. I have drilled a small hole at one end. I sit the wide bottom of the egg on the hole and shine the small pencil torch through the other end.  

Be VERY CAREFUL when handling the eggs as they will break easily! I use a plastic spoon to scoop the eggs up from the nest. Others use a long handled ladle type spoon. This can be done merely by bending a spoon head.   

The light will penetrate through the egg and if it is fertile you will see a network of red blood veins. This is the developing embryo!    

I use to check the fertility of the eggs and sometimes I still do on occasions. But after a while you acquire the ability to know from sight whether the eggs are fertile or not.