You can't really tell by the germ spot because all eggs have a germ spots. When the egg is laid you would need a microscope to determine if there was an embryo in the germ spot. It is only about 10,000 cells and just a small blob at this time. CJR is correct, you can definitely tell fertile eggs upon break out at 4-5 days in the incubator. By 2 days you should have your first small blood vessel visible.
Males seem to take longer than the females to become behaviorally competent to mate.
Don't expect a lot of fertile eggs until your males are over 6 months of age. A lot of males will be producing viable sperm before this, but they just don't seem to know what to do. At around 6-7 months something clicks in and they suddenly seem to know what to do. Some males will take longer than this and some will become active before this. You just have to watch for the foot stomping and mounting.