To test if your baking soda is still active enough to use, you’ll need some vinegar. If the baking soda fizzes, then it’s still active enough.
Testing your yeast is often also called “proofing” your yeast. To proof or test your yeast, you’ll need 1 teaspoon of granulated sugar and ¼ cup of warm water.
Fresh flour has a very mute, neutral odor, while spoiled flour may smell musty or sour. In addition, it might appear discolored.