How to check if baking powder is fresh?
Baking soda is sodium bicarbonate, a mineral. Left to itself in a clean and dry place, it will remain unchanged for billions of years. In particularly wet or polluted environments it may react with chemicals in the air and become unpleasant for use in food, which you can detect by smell.

Baking powder is a combination of baking soda and tartaric acid, which react in the presence of water. Eventually, even the moisture in the atmosphere is enough to cause this to happen. Checking whether your powder is still active is easy...just add a drop of water to a small bit. If it bubbles and reacts, it's still good.