As I've mentioned before, my mother's a great baker. Because of this, she never feels completely at home in a new place until she has all the ingredients necessary to throw together a couple loaves of bread, some biscuits, and a few cookies at short notice. So when she's vacationing in a place with a kitchen, she'll stock it appropriately, and proceed to bake up a storm for the duration of her residence. I've been lucky enough over the last few years to have her vacation near me, so not only have I enjoyed more baked goods than usual, I've inherited her vacation pantry once the trip was over. Win/win. And also, now, after several years of this, a home pantry with four cans of baking powder in it. This is three and a half cans more than would be reasonable given my rate of consumption. Even if I made biscuits every month, four cans of baking powder would last me slightly less than twenty years, if there were no science-fair volcanoes to contend with (or is that baking soda?).
I wanted to clear the decks in the pantry, so three of those cans had to go. However, I've heard that baking powder can get stale, so some experimentation was in order. Apparently, the way you test is to dump a teaspoon of baking powder into half a cup of hot water, and root for it to burst into fizzy action immediately. So I turned on the kettle, set out four little bowls and four littler bowls, and doled the teaspoons out into the latter (I admit to having a pretty comprehensive collection of small dishes, and to using them at the slightest provocation). I poured half a cup of steaming water into each of the little bowls, and then realized I'd need an extra limb for the next step: Fizz Race! I dumped the baking sodas into the waters as simultaneously as I could manage, and learned something right away: Baking Powder Lasts Forever. They were all promptly, extremely, identically fizzy.
So I just kept the one with the prettiest label (good old Rumford) and the moral of the story is this: if you have a nagging concern that occurs to you in the middle of the night, that your baking ingredients might be stale, and your tea cakes might be a little on the leaden side and it's just that no one wants to tell you, don't worry. Your baking powder's probably fine.
*when we were in Spain, we saw "fecha de caducidad" on a food package and, curious, looked up caducidad in the spanglish dictionary. The translation? Caducity. Gee, thanks. Well, once we got to an English dictionary, we learned that "fecha de caducidad" translates into the poetic "date of senility" or, of course, expiration date.