How can we tell if a turtle egg is rotten?

i have 13 turtle eggs that are fertilised, and i've tried to incubate them. They are about 1 month old, but i really want to know how to check them. You see, if i were to just leave them there, i would get the wrong hope of them all to be hatched. So i really want to know a way to check them, so if there really are rotten eggs, i would toss them away immediately, for it would not even hatch right? So please, i hope someone could help me with this question, thanks.