Why this matters
A baked good sets after gases expand, proteins firm, starches gelatinize, and moisture moves through the batter or dough. Ingredient ratios and pan depth influence all of those events. That is why a casual extra scoop of flour or a much smaller pan can change more than flavor. Measure carefully, then use color, aroma, structure, and temperature as doneness clues.
How to do it
Date baking powder and soda when opened and replace them according to the package. Test baking powder with hot water or baking soda with an acid when freshness is doubtful.
What to look, hear, or feel for
A fresh leavener foams promptly. Weak bubbling suggests it may not lift a cake, biscuit, or pancake reliably.
When something goes wrong
Do not compensate for old leavener by adding a large extra spoonful; too much can taste bitter or cause a rise-and-collapse. Use a fresh product.
Try it in a real meal
Practice on pancakes, muffins, quick breads, cookies, or one simple yeast dough before making several substitutions.