Cream of tartar is used to stabilize and give more volume to egg whites and is often mixed with baking soda to make baking powder. It's the type of ingredient that can sit in the cupboard for months or years until needed, and it lasts almost indefinitely when kept cool and dry. Just in case, here's how to check if it's still good in case you don't remember how old it is.
Check the expiration date on the jar, if available.
Open the container and take a look. The cream of tartar should appear dry and powdery. If you see clumps, it may have taken on some moisture and might not be effective.
Look at the color. This powder should appear clean and pure white with no yellow or dark spots. Dark spots could mean the cream of tartar has been contaminated and should be thrown out.
Shake a small amount of the powder into your hand and smell it. Fresh cream of tartar has a mild scent, but if it's old, there's no smell.