Acclaimed pastry chef and cookbook author David Leibowitz has a quick and easy way to tell if your baking powder has the ability to perform or if it is past its prime. He recommends putting half a teaspoon of the rising agent into a bowl, then pouring about 60 ML — about a fourth of a cup — of boiling water over it. If it starts to fizzle and bubble, the powder is still usable, but if it sits quietly, it may be best to replace it.
Because baking powder is made up of several components, Runaway Rice recommends that you shake up your can for 10 seconds, or to stir up its powdery white contents with a fork, to allow for the chemicals to mix properly. These components have different weights, and sitting around will give gravity a chance to pull the heavier chemicals down, particularly if you don’t use your baking powder often. The site says if you don’t mix the ingredients, you could be setting your ingredient up to fail the boiling water test, which takes no more than a few seconds to complete.