How do you tell a fertilized egg from an unfertalized egg?

To see the difference between fertilized and unfertilized eggs we can use a certain methods. In poultry industry, this process called candling. Basically in this process, we put the eggs in front of a certain light, so we can see through. We can practice this at home by using a bright flash light in a dark room. After we see through, the unfertilized egg's yolk will be seen as a dark round thing. But in fertilized egg's yolk, there will be some red thread like coming out from the yolk. This method can't be done directly after the egg is being layed. Usually we do this candling process after the egg is being kept in the incubator for 72 hours. I hope this help