To test baking soda for freshness, add 1/4 teaspoon baking soda to 2 tablespoons vinegar. To test baking powder for freshness, add 1 cup hot water to 2 teaspoons baking powder. Both should immediately begin to hiss and foam up. If not, discard the container and replace it with a new one.
Tip courstesy of Mastering the Art of Southern Cooking by Nathalie Dupree and Cynthia Graubart