I'm using a flashlight, toilet paper roll to direct the light and going into a dark room. The method tells me that it's fertile, I'm not saying I can tell if it will develop because I've had duck eggs that don't have an embryo in them at all but they were fertile. How I know you might be asking because I've seen my drake breed the hen and he's only breeding one hen and every egg I get from her is fertile.
You are not getting it. If you were to worm your birds, it will leave them infertile for a while. However your birds do not know this so they will continue to mate and the hens will continue to lay eggs, but those eggs will not be fertile. If you candle those eggs how would they look different than the fertile ones?