Depends on the crop, etc. Some are only good for a year, some longer. It sounds like you mean seeds from a dry packet, rather than seeds you saved from your own harvest. They might have an expiration date.
If you keep them dry in a sealed container in the fridge, that should help a little bit. I have tried to use old leftover vegetable seeds with very mixed results. But there's no harm in trying, right? At most, you will waste some potting soil and a little water for the seeds that never germinate.
As Steve says, Kathy, commercially purchased seeds should have an expiry date on the package. Vegetables seeds will generally last much longer than flower seeds, but their viability does depend on how they were stored. In an airtight container in a cool, dry place is the gold standard. If you want to test them, space several out on a few layers of moist paper towels, roll up so that the seeds don't touch, and enclose the bundle in plastic wrap so it doesn't dry out. Place in a warm bright location (65 to 70 degrees is fine) but away from direct sun. Check the seeds every couple of days. If they haven't germinated - or only a few have sprouted - in a couple of weeks, chances are they're no good. If you want to just try using them this year, sow them more heavily to allow for lower germination rates.
Douglas must be my age. Way back in the early 1980s, I was reading an article written by Dick Raymond that used that method way back since he was a kid in the late 1950s. Practical advice NEVER fades.
To get technical and to see if it's worth still using the seed anyway just count out ten seeds and use Douglas' suggestion. Keep track of the number (if any) that germinates. If 5 of 10 survive... You may want to just buy new seed. If 8 of 10 germinate, I say use it and throw down a little more to make up for the 20% average that probably WON'T germinate.
Farmers way back when were no FOOLS when it came to crops coming up or not coming up... It was their lifeline.
The seeds should last for more than a year if they are kept in a coo, dry environment. There are also ways to test their viability and whether they will germinate or not.