Lay a piece of damp kitchen towel on a plate, then sprinkle a sample of your old seed in individual rows to aid identification.
Cover the plate with clingfilm and keep it in a warm place indoors. Make sure the towel stays moist and check regularly for signs of germination, noting the date when shoots appear.
Germination times vary between veg, but after two weeks most viable seed should have sprouted. Count how many have germinated – if it's about half, then you've got 50 per cent viability. The lower the percentage, the more seed you'll need to sow to get a decent crop.