How to tell if an egg is bad

There are two main ways to check if you have a bad egg.

There is a small air pocket in the large end of the egg. When the egg is fresh, the pocket is only about 1/8th of an inch deep and as big as a penny. As the egg ages, however, it loses both moisture and carbon dioxide, shrinking it, so that the size of the air space increases. And the size of the air space determines the buoyancy of the egg.
So if you submerge a very fresh egg in water, it will lie on the bottom. An egg that is a week or so old will lie on the bottom but bob slightly. An egg that is three weeks or so old will balance on its small end, with the large end reaching for the sky. And a bad egg will float.

on the other hand,

you could just look for the evil grin.