First, check the sell by date on the carton. Eggs will keep for several weeks beyond that date. It is reassuring to know that refrigerated eggs rarely spoil, but their quality decreases. You can tell an egg is less fresh than you'd like if it becomes very runny. The white should be thick and viscous.
To tell if an egg is of top quality without cracking open the shell, place it in a bowl and add water. The fresher the egg, the flatter it will lie. As it gets older, the egg will begin to stand up, and when it's really old, it will float.