Most serious bakers have a small jar or canister of cream of tartar in their pantry, though in most cases it's seldom used. It's a powdered form of tartaric acid, a byproduct of wine production. Bakers find it handy because it's one of the few acidic ingredients available in powdered form, so it's useful for whipping egg whites or ensuring that baking soda can produce its full amount of lift. Cream of tartar isn't perishable, so usually it's still good as long as it's uncontaminated.
Inspect your jar or can of cream of tartar, looking for any visible leaks, rust, condensation, cracks or punctures. If you find any visible damage, discard the contents and buy fresh.
Open the container and visibly inspect the contents. If there are visible signs of moisture, or if it appears to have drip marks inside, discard the contents.
Shake the jar to ensure that the contents are still a loose powder and haven't settled into a solid lump. If the tartar has solidified but shows no signs of outside contamination, it should still be safe to use. This is especially common in areas of high humidity. Break it up in a spice grinder or by using mortar and pestle, or discard it and buy new if you prefer.
Test the potency of the cream of tartar, if you're uncertain, by stirring a half-teaspoon into a half-cup of warm water. Add a generous pinch of baking soda. If it foams vigorously, the soda and tartar will still work together effectively in your baking.
If you're using the water test and don't get a reaction from your soda, repeat the test with lemon juice in the water. If the soda still doesn't foam, it's your soda that's the problem. Soda is much more susceptible to moisture in the atmosphere and loses its potency over time. Date your boxes of soda and replace them after a year if necessary.
If your cream of tartar isn't usable and you don't have time to go to the store, substitute other acidic ingredients. Lemon juice or vinegar work just as well for whipping egg whites. The crystals sold at supermarket produce sections to keep cut fruit from browning are powdered ascorbic acid or citric acid, and in an emergency you can use those in your baking to activate the soda. Alternatively, use buttermilk or other liquid acids.