Once opened, baking powder has a shelf life of 9-12 months. It can be tested by stirring 1/4 teaspoon of baking powder into 1/2 cup of warm water. If bubbles begins to form, and the water begins to cloud, then your baking powder is still fresh!
Some people say that baking soda has an indefinite shelf life. And while that may be true when using it for cleaning, it certainly is not true when using it for baking! Pay attention to the expiration date on the box. Baking soda can be tested by using the same method as above, but add 1/4 teaspoon of vinegar to the water before adding the baking soda. If bubbles appear, then the baking soda is still fresh, and you’re ready to bake!