Omega 3 Eggs
Phofiend, if the egg is fertilized and has not been incubated, you really can't tell. If it has been incubated for 24 hours or so, it has a small white donut shape on the yolk--about the size of a lentil, or a little smaller.
Longer than that, and blood veins appear--the yolk begins to look like a bloodshot eyeball.
If you are seeing a bloody spot or small fleshy colored blobs, those have nothing to do with fertilization--they are just signs of slight damage to the hen's reproductive tract.
All of these eggs are perfectly edible, but may be a little esthetically off-putting.
In savory dishes that have other strong-tasting ingredients such as onion, no. But I recently made a creme brulee that tasted weird. Then I got the same off flavor in a plain omelette, and I realized it was the high omega-3 eggs. I bought another dozen of a different brand to make sure they weren't just "off" and sure enough, the same taste. I now buy free range eggs from a local poultry farm. They taste great, but occasionally, I get a fertilized one. A bit yucky, but definitely preferable to funky.