Dried herbs and spices rarely go "bad"–as in make you sick if you use them–but they certainly lose their flavor over time. To prevent a pileup in your spice drawer of flavorless seasonings, label the bottles when you bring them home. A tiny piece of masking tape on the lid or label that marks the month and year–ie, "9/2011" tells you to ditch the flavor-faded remnants after a year or so.
To check the dried herbs and spices in your cabinet, rub or crush a little between your fingers. If the aroma is faint, chances are the flavor will be, too. Color can also be an indicator: as color fades, so goes flavor.
For more tips, see Get the Best from Spices: Storing, Buying, and More