the USDA says:
Are Fertilized Eggs More Nutritious?
No. There is no benefit in eating fertilized eggs. There is no nutritional difference in fertilized eggs and infertile eggs. Most eggs sold today are infertile; roosters are not housed with the laying hens. If the eggs are fertile and cell development is detected during the candling process, they are removed from commerce.
and the canadian egg marketing agency website says:
Can I eat a fertilized egg?
Yes. Most eggs sold today are infertile because there are no roosters housed with the laying hens. But fertile eggs can be found at roadside stands or health food stores. There are no nutritional differences between fertile and infertile eggs.
If fertile eggs are not incubated, there will be no development of the embryo and no way to distinguish them from infertile eggs. If fertile eggs are properly incubated for a few days, development of the embryo should be visible when the eggs are candled, and federal regulations prohibit their use as human food.
it looks as though in order to tell if the eggs really were fertile, they (the producer) would need to incubate them and then candle them, and then on finding them to be fertile, they'd be illegal to sell as food. so they'll have to be sold as 'a rooster was present, so some of them might be fertile, but they aint been incubated so won't have developed and so we can't tell if they actually are fertile' right?