There is no easy test for determining the acceptance of baking powder beyond the use-by date on the bottom of the can. If your baking powder has been opened for more than 6 months, it may not have enough leavening action to leaven your recipe. You can test a small amount of powder in warm water to see if it fizzes, however, this is not a definitive way to test your powder. The baking powder may have enough leavening action to react in the water, but may not have enough to leaven a cupcake. If you are not sure how long your can has been open, we recommend replacing it so that you can avoid preparing a recipe that doesn’t leaven.