A question about a recipe: Brown Sugar Apple Upside Down Cake with Apple Cider Caramel and Spiced Walnuts

I wonder if there's some way to store baking powder so that it doesn't "oxidize" -- i.e., isn't stored in contact with air. Maybe a zip-loc with the air pressed out?

Try "proofing it" just by putting a 1/2 tsp in a cup water and see if it fizzes.
Also, do that when you first get a fresh unopened can when you open it so you have a baseline visual or amount of fizz it should have.
I usually keep unopened baking powder on hand as a back up. Even with the plastic lid with high humidity etc it starts to degrade faster than the 'use by/exp' date once opened and exposed to air.

There's no question it can go off - I made some scones with some expired baking powder a few weeks ago (I don't bake all that much) and the results were disappointing to say the least :)

I like to use baking powder within 12 months. Does the packaging on yours have an expiration date, or do you know how long you've had it for?