At the moment you don't have the possibility of that problem, once they start laying, if you don't have a Roo, your not going to have that issue, if you do have a roo, just make sure you collect the eggs everyday so that there isn't the possibility of a chick developing. Short of cracking the egg, I don't know how you could tell, I guess if you know what your looking for you could try candling, but I don't think that would work till after the egg has been in incubation for a week.