Eggs are not something that I have go bad on me very often. But if you are questioning whether an egg is still good or not try this trick.
To determine whether an egg is fresh, immerse it in a pan of cool, salted water. If it sinks, it is fresh - if it rises to the surface, throw it away.
Also if you are unsure an egg is bad and you are going to use it in a recipe crack it into a seperate bowl first. This way if it does turn out to be bad you only have to throw the egg away not the entire dish. This is also useful to prevent any unwanted shell in your dish.