Probably a pretty good assumption. Do you know how to tell the difference between a fertile egg and an infertile egg when cracked into a dish? You could watch eggs that you're cooking for a few days to see how many are actually fertile.
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That's exactly how I keep track of fertility. I just look at the white spot on the yolks, in my eggs that I use for cooking, and see if it's a blastderm or a blastodisc. Pretty much every egg I open is fertile, so I figure it'll be no problem to hatch a few when I want to.