Commercial baking soda and baking powder will last for quite a long time on your cupboard shelf, but they will eventually lose their potency and expire. If you're wondering if yours is still good, it's best find out with a quick test before baking rather when your muffins emerge from the oven dense and flat. Here's how!
For baking powder, all you have to do is stir about 1/4 teaspoon of powder into 1/2 cup or so of very hot water. Tap water is fine, and the proportions aren't fussy. The soda will immediately start to bubble if it's still good.
Baking soda needs an acid to get a reaction, so use the same method as for baking powder but add 1/4 teaspoon of vinegar to the water before water before adding the soda. As before, if it bubbles up, your soda is fine to use.
If you don't see bubbles, then sadly, you'll need to take a trip to the store and buy a fresh box! If there isn't much fizzing, your ingredients are likely on their way out. You can probably still use them to get some lift, but your baked goods will be more dense that usual. Don't be tempted to double the amount of baking soda or powder - that would make your food taste seriously strange!