Hens lay fertile eggs within about 3 weeks of being ‘visited’ by a rooster. She then doles out the semen one egg at a time as she lays a clutch. After laying as many as she wants to incubate, she sits on them, keeping them uniformly ~99° F for 21 days.
Fertile eggs are indiscernible from infertile eggs and can only be proven one way or the other by incubating or cracking them open. Only a trained eye can tell the difference, even by cracking and examining. Noticeable development is only present after 5-7 days at incubation temperatures.
Eggs are generally collected within a few hours of being laid, so there’s normally no danger of finding any development, as they aren’t incubated.
TL;DR: Farmers collect eggs regardless of whether they are fertile or not. Fertilization is absolutely irrelevant, as a normal person couldn’t possibly discern whether the uncracked egg was fertile until day 8 or so of intentional incubation. They look and taste the same either way.
*Somehow deleted part of my response while trying to fix a typo, sorry!
As an additional note to what others have said, many farmers have a variety of chicken breeds on their farms. Some breeds of chickens instinctually are more attuned towards nurturing eggs by sitting on them. Other chickens have been bred to just pump out eggs and will not sit on their eggs, thus, even if they are fertilized they won’t grow up to become new chickens.
So in a way, another method of controlling reproductive eggs vs eating eggs is through the choice of chicken breed.
I know this from working at an organic farm, but even there, they had incubators to ensure certain eggs successfully led to new chicks being born.
Farmers try to control this by not allowing the chickens to mate except in specific circumstances, but this isn’t always a perfect process: preventing chickens from mating can be harder than you might think.
As a second line of defense, they use a technique called candling to detect embryos. This involves holding the egg up to a strong light and looking through it: it’s not *quite* as simple as the process you may have seen in cartoons, but it’s pretty close. It takes some skill to read what’s going on inside the egg, especially at early stages, but you can get a decent read on which eggs are fertilized and which are not.
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