Chicken farmer, here...
If you buy factory (battery) eggs, there's almost no way to get fertilised eggs. We sometimes get roosters in shipments, but we hunt them out and separate them ASAP. Blood specks are not permitted past the candler. If you get some, though, it's usually no problem to eat them... and not usually indicative of an embryo.
We used to get people panicking about 'bugs' in our eggs. These people were buying 'cracks' very cheaply from the farm... and we had a buckwheat system. Well, we don't sell cracks anymore. To the public, anyhow...
Eggs keep a long time, even unrefrigerated (though we don't recommend this). Checking them is easy. Shake the egg. There should be no movement inside the shell. None.
If there is movement, throw away the egg.
But yes, with free-range eggs, you can get a lot of interesting things. A quick candling should eliminate the ones with blood spots/young chickens/whatever else...