What do the other 6 look like? If they are clear inside with just the yolk and aircell visable after day 5 of incubation then it is pretty safe to say they are infertile. At day 5 you should see a network of vessels inside of the egg and a heartbeat. When the eggs are fertile you will know they are fertile at this time, if they are not then they will be clear. Also if you look at the shells of the eggs, an infertile egg will have a slight pink cast to it and a fertile egg will be stark white.
Here is a pic of an infertile egg:
Here is a fertile egg at 8 days into gestation:
A comparison of an infertile egg, a fertile egg, and a nonviable egg:
Thanks again Susanne (srtiels) for creating such wonderful pictures! You have no idea how much they help me and others!!