How will you identify fertilized egg from unfertilized egg?

The only way without any incubation is by cracking the egg and in the yolk there will be a small white dot, sometimes barely noticeable, and sometimes it can be grey or even black.

The other way to tell is by incubating the egg for at least 6 days and then candling the egg which is done by done by using a bright light against the egg so you can see and it has veins all over. if there is no veins by 10 days it would be fertile at all.