I have looked for the bullseye, and I just can't see the difference between fertile and infertile eggs. I have read descriptions and looked at photos. I have looked at eggs when the flock is all hens, with no rooster present for months. And I have looked at eggs when there is an active rooster and every egg that goes in the incubator is fertile. And I cannot reliably see a difference.
But I have no trouble seeing the embryo when I candle at 4+ days of incubation, so that works better for me. (After only 4-5 days, the clear eggs are probably safe to eat. Since I have only incubated eggs when I had a surplus, I have cooked the clears for the dogs and chickens to eat, rather than eating them myself. But I'm keeping it in mind in case I ever have an egg shortage at incubation time.)
For people who can see the difference, checking the eggs you would eat anyway is a great way to check fertility-- it just doesn't work for me.