Don't trust the expiration dates on supermarket coffee. If you're unsure of whether you're really getting fresh-roasted coffee for the best possible cup, use a zipper-lock bag for an overnight test.
Photo by pokpok313.
If you don't want your coffee to taste like the last pot at an all-night diner, you can take a lot of steps to brew the best cup without spending a bundle. Unless you're roasting yourself, though, you're at the mercy of whoever's selling you your beans. If you're unsure the quality, or how long you've had them stashed in your cupboard, Cook's Illustrated has a good test:
To check if your beans are fresh, scoop 1/2 cup into a zipper-lock bag and press out all the air, then seal the bag and leave it overnight. If the beans are within seven to 10 days of roasting, they will make the bag puff up from the carbon dioxide that they release. If the bag remains flat, then the beans are not producing gas—a sign they've passed the point of peak freshness.
If you're a coffee snob, in practice or at heart, you should make every effort to pick up the October 2009 issue of Cook's Illustrated. Some of the basics will be familiar, but a two-page spread on "Secrets to a Perfect Cup of Coffee" is stuffed with testing tips like the one above, plus equipment reviews, step-by-step illustrated guides to using a French press or manual drip system (their preferred method), and best practices for buying, storing, and brewing coffee. If nothing else, you'll learn how to dismiss coffee as "having less than 18-22 percent of its soluble solids extracted," rather than just "weak"—best said with a monocle and fancy hand flourish, perhaps.
eHow picked up on the overnight bag test, and has a few additional tips on bean longevity as well, if Cook's just isn't in buying range.