A lot of people rely on the date on the packaging to tell them when food has gone bad, even with eggs, but the "sell by" dates are often rather arbitrary, and do not correlate to expiration dates. If you've been tossing away your eggs based on the dates on your carton—you're wrong.
Your eyes and nose are the best tools for determining freshness with meats, produce and herbs, but you can't really use your senses to test an egg before you crack it (unless you're highly skilled).