if you have a rooster to every say 10 hens [depends of the breed] then a good guess is that all your eggs are fertile. the odd time one may not be, not a problem really, put the eggs under the broody hen, seperate her from the flock perferably in a quiet safe place with wheat and water, and wait 21 days. then you will know how many eggs are fertile, if the egg doesn't hatch, shake it gently and if it sloshes, throw it away. but give her 48 hrs to hatch out. some eggs take a little longer and some are a little shorter. as the chick develops iin the egg the egg seems to get heavier. no chick and it stays light. if you have tomany roosters per hen, the hens hide and then you may not have all fertile eggs. people are strange, we get a lot of people will only eat LARGE BROWN eggs. sometimes i ask if they eat the shell, not sure of the difference between brown and white calcium hummm..... smaller eggs are the tastiest. i like the pretty blue and green ones but never eat the shell, lol just eye candy .