We keep hens and have 2 roos so all our eggs are fertilised. The only way to tell if an egg is fertilised is to crack it and you may see a faint bullseye marking in the yolk. That's it. For there to be a developing embryo that egg will need to have been sat on for days and days, so high unlikely in a commercial farm where eggs are collected daily for selling. Even we collect daily otherwise there isn't space in the egg boxes for the hens to lay or it can encourage rats.
If you think about it, it takes 21 days for an egg to go from fertile to chick. That's 21 days of the hen sitting for about 23 hours a day. We candle on day 6 and at that point you can see a few veins around the yolk, which is still yellow and yolk like. By day 12 when we candle again you can start to see some development of something embryo like. So for people freaking out about commercial eggs being fertile, they won't look any different unless they've been under a broody hen for at least 6 days, and there's no way a broody hen would go unnoticed in a commercial set up. So it's very very unlikely you will crack an egg from a supermarket and find a half developed chick inside!
We sell at the garden gate and eat a lot of eggs, in several years I've only once cracked an egg with veins and that was a suspect one I'd found under a bush and couldn't be sure how long it had been there. Ducks are a whole other matter as it's very hard to sex commercial egg laying breeds and lots they refuse to lay in nest boxes!