It's easy to check an egg for freshness. Put it in a bowl of water. If it lies flat on the bottom, it's very fresh. With the passage of time, air enters the egg, and the broader end will start to float.
Another way to look at it is after you've cracked the egg open. See how part of the white of this fresh egg is almost gelatinous, and the yolk is nicely rounded:
After two weeks, eggs are noticeably less fresh, though still usable. The USDA recommends using eggs within 3-5 weeks of purchase.
According to the USDA:
An egg can float in water when its air cell has enlarged sufficiently to keep it buoyant. This means the egg is old, but it may be perfectly safe to use. Crack the egg into a bowl and examine it for an off-odor or unusual appearance before deciding to use or discard it. A spoiled egg will have an unpleasant odor when you break open the shell, either when raw or cooked.
Want to learn more? Head over to Mother Earth News to read an interesting article on an elaborate multi-month egg preservation experiment.