Most seed packets state the year of intended use, usually on the top or bottom flap, and many also list minimum germination standards and usual seed life. That's a good place to start when determining whether or not you can use seeds from past seasons.

Seeds that have been stored in a cool, dry place are generally viable for at least a year past the season for which they were packaged and may be good for up to five years.

See some related articles below for more information.