A fertilized egg's white spot on close examination will look like a donut or a target in the early stages. It won't be solid, flat looking. That's an easy way to tell if your egg is fertile.
(And, sometimes, eggs sold as fertile are not so. The flock where the egg comes from has a rooster is basically what "fertile" means on the egg carton.)
(And, sometimes, eggs sold as fertile are not so. The flock where the egg comes from has a rooster is basically what "fertile" means on the egg carton.)