> It is rather obvious when they are.

Commercial egg-laying hens aren’t going to be let anywhere near a rooster, so you’re not going to get fertilised eggs from the store.

But… unless they’ve been incubated, the only difference is a white spot in the yolk.

If you get eggs from a recreational / family farm, you may well get a bunch of fertilised eggs — you wouldn’t notice unless you knew what to look for.