"How do I know they are fertile? Is there any way for sure of telling"
well, that may be tough if you've never seen fertile and infertile eggs. infertile eggs are typically more yellow in color, and have more of a scaley texture to them. fertile ones will be a bright chalky white with a smooth shell.
the way i will usually tell is take the egg length wise between my thumb and pointer finger and squeeze it a bit, infertile ones will usually be squishy, and fertile ones will be much harder.
this will usually only work the first few weeks after the egg has been laid because the egg will start to absorb moisture and become sort of squishy anyways.
if there's any chance i think it is fertile, i incubate it. it will start molding within 2-3 weeks if it is infertile.