I’m not sure what your concern is. I agree, the only realistic way for us to tell if they are fertile is to crack the egg and look for the bull’s eye. If most of the ones you crack are fertile, most of the ones you don’t crack will be fertile. That’s handy if you want to hatch some eggs. When you crack the egg, the bull’s eye might be on the bottom. I gently turn the egg with a spoon when that happens.
A fertile egg will not develop unless it is stored warm. It can develop some if the egg temperature is in the 80’s Fahrenheit, with the warmer it is the faster it develops. Even if it is under a broody all day though, it will not develop enough to be anywhere close to noticeable as long as you gather the eggs daily, preferably at the end of the day. So just collect them daily and keep them relatively cool after you collect them. I store them at room air-conditioned temperature.
There is no difference in taste or nutrition in a fertile versus unfertile egg. The only appearance difference is the bull’s eye, and you have to know what to look for to see that.
