Put the egg in a glass with water. If it floats it is rotten, if it lies down on the bottom it is good. I've been using this method since ever so only once I felt the smell of a rotten egg. Yuk!
Explanation from To your good health:
"As eggs age, gases build-up inside their shells, making them more buoyant. This is the best way to test whether an egg has gone rotten without having to break open the shell, risking the foul odor escaping. When an egg is extremely fresh it will lie on its side at the bottom of a pot of water. As it ages, the egg will begin to point upwards, and will finally float completely when it has gone bad. The whites of fresh eggs are firm, while not-so-fresh eggs have a very watery white. This is why it is best to use the freshest eggs possible for poaching and frying. Older eggs (before they actually float) are perfectly good for omelets, scrambling, or baking."
