How to tell if an egg has gone bad

Bad eggs can be a nasty affair, in both their smell and how they affect the stomach (it isn't good). There are a few "warning signs" for bad eggs that you can keep in mind.

Inspect the shell. Even relatively fresh eggs can easily spoil if there is a small crack in the shell. If the shell is cracked or holed in any way, and you are not absolutely sure it happened that same day, immediately discard the egg. This holds especially true for eggs that have any white dried onto the outside of a crack.

Use a spoon to drop the egg(s) into a bowl of water, at least twice the depth of the (largest) egg's height. If, after a few minutes, the egg has bobbed to the top, it's spoiled. (For those that need to know why : the gasses emitted an an egg spoils make it boyant.)

If it has been more than a week since you purchased the eggs, break them individually to avoid a single bad egg from contaminating a good set. Look at the yolk (it should be a bright yellow, not a washed out or faded color), and do a sniff test. If the egg looks and smells good, combine it with the others; otherwise, put it in a sealed plastic bag and throw it away.  

There are some things in eggs that often make people think they have gone bad, or are already fertilized.

An opaque, stringy white mass is sometimes confused as a fertilized egg. This is a normal part of the yolk, is visible to one degree or another on all eggs, and eggs with this shouldn't be considered unusable.

Black or red spots on the yolk are the result of broken blood vessels when the egg is formed. Again, these are perfectly normal and safe.

Color-tinged or puffy egg whites are usually just a formation defect. If the egg appears to be in otherwise good health, there's no reason to discard these eggs. 

If you purchase your eggs from a small egg farm, don't be alarmed if, from time to time, you run across a fertilized egg. You'll know fairly quickly by a (relatively) large, dark mass attached to the yolk. Personally, I've only had this happen once or twice, when I received eggs from friends.