Unless you are an avid baker, it can take a long time to use up a box or can of baking soda. If you can't quite remember when you purchased and opened your container you may want to test its leavening ability.
Have no fear! It only takes a moment to check your baking soda. All you need is a glass, ½ a cup of warm water, ¼ teaspoon vinegar, and ¼ teaspoon baking soda.
Add water and vinegar to a glass and stir. Add baking soda to the glass. If it fizzes you are good to go, the leavening agent is still active. No fizzing - it is time to get a new container of baking soda.