I used to think baking soda and baking powder were eternal, like the culinary equivalent of Styrofoam or crayons. But as with most things, even baking soda and baking powder have a shelf life, as they both will become ineffective after a year or more. Baking powder will usually (depending on storage conditions and humidity) last about a year once opened, and baking soda (also if conditions are optimum) will last about 18 months. But who really remembers when they purchased that last box of baking soda or baking powder? Now that we are deep into baking season, it would be helpful to have some way to determine what is good and what is garbage.
Of course there is a trick to test the efficacy of both baking soda and baking powder. As demonstrated by kitchen wiz Suzy Brannon at Chow.com, baking powder can be tested by adding a ¼ teaspoon of baking powder to a ½ cup of boiling water. If it fizzes, it is good. If it does nothing, than it is not worth using. Similarly, testing baking soda requires ½ cup of boiling water with ¼ teaspoon of baking soda and ¼ teaspoon of white vinegar added (you need an acid to test its strength). If you get fizzy action, then you are good. If you get nothing, then it is time to trash it, unless you want a dense and puck-like cake.
On the next page, you can find two videos demonstrating each technique.