How can I tell whether my baking powder is fresh?

Check the date of manufacture first; if it's past the use-by date, then it's no longer fresh. To test the baking powder if you cannot find a date or suspect it's gone off anyway: Pop 2 teaspoons of the baking powder into a cup of hot water from the tap. If the powder foams up and bubbles or fizzes straight away, it's still good to use. If this reaction is slow to happen or doesn't happen at all, the baking powder is no longer viable and should be thrown out. After opening baking powder, label it with the date: it's good for 6 to 12 months after this date and then it should be thrown away.