Regarding the term “properly stored” — that shoebox on the shelf doesn’t really qualify. Seeds retain their viability longest if they’re kept in a cool (32 to 41 degrees F.), dry place out of the sun. Gardeners are often advised to store extra seed in a glass or other airtight container in the refrigerator or freezer, with a packet of powdered milk from a freshly opened box, or absorbent silica gel (sold by craft stores to dry flowers), in the bottom of the container to extract extra moisture. A tablespoon or two of powdered milk per glass jar will absorb excess moisture for about six months. Do this with the seeds that you order this year, and you may get more for your money in the long run. (You can put more than one packet of seed in each jar.)
If you’re in doubt about existing seeds, try germinating 25 or so on a damp towel or paper towel. How many send out roots within a week or 10 days? If the percentage is quite low, order new seeds. If about 50% of the seeds germinate, you might want to use these up this year, sowing them thicker than recommended, and order new ones next year. If germination is good, you’re all set for this year, and maybe more. Remember, though, that seeds will germinate more easily on a constantly moist towel than they will in the ground, where they’re subjected to more stresses.
Once you know or make an educated guess as to which seeds are viable, cross them off a checklist and you’ll be left with names of plants for which you’ll need new seed. The index on the back of the Fedco catalog can serve as a checklist; or you can download this list and use it as is or adapt it to the kinds of flowers, vegetables and herbs that you like to grow.