So, now you have some dry yeast (rapid or regular) that you've stored beyond the "Best By" or "Use By" date. Can you use it?

Yes, if it's still alive and hasn't grown too old to perform.

How do you know if it's still alive?

Measure out 1/2 cut of warm water in a 1 or 2 cup measuring glass and stir in 1 tsp of sugar. Add 2-1/4 tsp.s of yeast and give it a stir. In about 10 minutes, the yeast should have foamed up to at least the 1/2 cup mark.

If your yeast are alive and well, use this yeast right away in your recipe (and remember to deduct 1/2 cup of water from the recipe!).