I came across a way to test whether seed is still viable. Dampen some paper towel, and sprinkle a few seeds from the out-of-date packet across it. Roll it up, put it in a plastic bag and keep it somewhere warm (like on the top of the fridge or some electronic equipment that stays on). Check after two days, then at least once a day after that, making sure the paper doesn't completely dry out.
If half of the seed germinates, plant more closely than usual for the same yield.
I guess you could also plant the germinated seeds (still on the paper towel). Even if nothing germinated, I'd probably throw it all in the garden anyway, just in case!