If you take a pinch of the baking powder and put it in water, it should fizzle. If it does not fizzle, it is no longer fresh.
Baking powder leaves more residual and "cakes" together. Corn starch is a little bigger and doesn't taste like baking powder.
For a good long time if it is kept properly stored. Here is a test you can use to make sure your baking powder is usable . Put water in a teaspoon and sprinkle a little bit of the baking powder in the water. If your baking powder is good the water will fizz sort of like a soda does. if you don't hear or see the fizz then baking powder is no good and should be tossed out.
Baking soda is sodium bicarbonate used in leavening foods that have acid in them. Baking powder contains baking soda plus an acid, and is used in leavening acid-free or low-acid foods.
Baking Powder
Baking powder in a recipe helps your item rise when baking.
baking powerder is baking powder is not the same!
baking powder
Your baking powder might be old. Check the date on the can or just buy some fresh.
Baking soda does not rise as well as baking powder
You tell me. What happens when you mix baking powder with vinegar
Baking powder is an acid
baking powder is neutral