How long are old garden vegetable seeds still usable? Packed for "--" season?
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. Here is an article "Give seeds a test for A+ performance" that lists average seed life for common vegetables and flowers. The article also includes an easy germination test you can do to check your seeds before planting.