Fertile? See the birds mating--most likely fertile. Otherwise, cannot be sure until set and then candled after 4-5 days with light shells. See a red spider, then a dark blob for a few days, then cloudy. Start all eggs at the same time, so they hatch a the same time. Eggs keep for days...
I have NEVER had a rotten egg in more than 50 years of poultry. This includes eggs in nests that hens"stole" outside, well hidden, and after being in the slightly protected nest, in rain and cold night, warm or hot days from egg 1 and more than 2 weeks more, as each egg was laid, then under...