Here is an old trick to test an egg and get an approximation of its age.
Simply lay the suspect egg into a bowl of cold water.
If the egg:
A. Sinks to the bottom and stays there, it is very fresh.
B. Sinks, but floats at an angle, it's more than a week old.
C. Sinks, but then stands on end, it's about two weeks old. This is getting to the point where you might want to use these eggs in recipes rather than cooking them on their own, as they don't taste as good as fresh ones.
D. Floats, it's too old and should be discarded.
Eggs act this way in water because of the air sac present in all eggs. As the egg ages, the air sac gets larger which makes the egg float.
This is a great tool for the homesteader. At times you may find an egg that wasn't layed in the usual location. Because of this, you may not always be sure that the egg is fresh. This old trick is more pleasant than cracking open an egg that's been sitting in the sun for the last four weeks.
As I mentioned, this is a very old method. In fact in 1750, Hannah Glass gave this advice to cooks -
a "way to know a good egg, is to put the egg into a pan of cold water; the fresher the egg, the sooner is will fall to the bottom; if rotten, it will swim at the top."