You've got a pile of seeds from last year, or maybe three years ago. Which ones are still good?
Most of them are. Most seeds last for 3-5 years when kept in a cool, dark location. I keep mine in a box in the dining room cabinet.
The only seeds that lose almost all viability after one year are chives, onions, leeks, and parsnips. (I’ve forgotten to harvest parsnips in a container and they went to seed the next season, so I just saved them. I did genuinely nothing in this equation, so it’s very easy to save parsnip seeds.)
Corn is surprising, lasting only 1-2 years. I grew some heirloom corn last year with seed that was at least three years old. The highly bred sweet corns are probably the ones with a short shelf life.
All the rest are said to last between 3 and 5 years, as seen by this High Mowing chart
If you are unsure about the viability of your seeds, you can test them by submerging them in water or wrapping them in a wet paper towel for 24-36 hours. (Make sure the paper towel stays wet). If you see any signs of life in that time (white roots emerging or seed husks swelling) they're still good. If not, it may be time to get new seeds.
What, you’re not bored yet? You want to hear about some really long lasting seeds? Ok, if you insist.
I've heard stories of tomato seeds germinating at 15 years old. Again, these are probably heirlooms.
Beans, too, last quite a long time. One time I was planting beans at my aunt's farm and ran out of seed. She went to the shed and gave me a worn out packet and said to keep planting. When I asked how old they were, she said they were ten years old and seemed unconcerned about it. I didn't stop back in to visit them, but I'm sure they grew.
Those are my seed stories.