Store baking powder in a cool, dry place. Once opened, baking powder should be fine for up to 6 months. Be sure and keep it tightly sealed after opening. It’s very perishable so, unless you use if often buy it in small cans. If you are unsure of its potency for some reason, test it by combining 1 teaspoon of it with 1/3 cup hot water. If it bubbles energetically, it’s fine.

Additional Resources:

http://www.joyofbaking.com/bakingsoda.html