Here's a simple test: Place the egg in a bowl or pot of water. If the egg floats it is bad. If it sinks to the bottom it is good, if it stands at a 45 degree angle it's still OK. However, standing on end probably means that the egg is too old.
This works because the air space in an egg increases as it ages. Thus an older egg will have more air and will float. A fresh egg will have very little air and will sink.
Source: The Joy of Cooking (Wow - I never really thought I'd use this book!)
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