Cream of tartar is the type of ingredient that can sit in your cupboard for months or years until you need it, and it should stay good almost indefinitely if it's kept cool and dry. It is used to stabilize egg whites and give them more volume and is often mixed with baking soda to make baking powder. Cream of tartar typically doesn't go bad, but you can still test it for freshness.
Check the expiration date on the jar. Some jars and canisters may not have an expiration date.
Open the container and look at the contents. The cream of tartar should look dry and powdery. If you see clumps, it indicates the cream of tartar may have taken on some moisture and may not work.
Look at the color of the cream of tartar. This powder should appear clean and pure white, with no yellow or dark spots. If you see any dark spots, your cream of tartar may be contaminated and you should throw it 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, you won't notice any scent.