You can candle them, but you can't always tell anything for sure for the first week or so. Even then, I recommend incubating all eggs just in case. A fertile egg will candle pinkish in hue and you will be able to see a pink circular shape on one side of it--that is the embryo. An infertile egg will candle yellow and will not have an embryo. That said, sometimes fertile eggs look yellowish for the first week or so before the embryo starts to become visible. I have also had eggs that I thought were infertile so didn't keep a close eye on them, but they ended up hatching out perfectly good babies. So, I just incubate everything just in case.