Because of their importance, checking and testing these valves helps ensure that the consequences of a broken check valve are never realized.
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Shut off the water supply to the check valve. Follow the water line backwards (the flow of water is indicated by an arrow on the check valve) and switch off the next valve. Wait 15 to 20 minutes for the water pressure to subside.
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Check the valve for debris by tapping its side with the handle of a screwdriver. A rattling sound means the check valve is fine, but inconsistent grinding or squeaking indicates that the valve is stuck on some form of debris. If the check valve is of the 90 degree (right angle) variety, remove the top of the valve (use the channel locks to turn the top of the valve to the left) and check the inside of the valve directly. There should be a dropping ball-pin on the inside, but nothing else. Replace the top of the valve.
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Stand near the valve and have an assistant turn the water supply on for five seconds before quickly shutting it off again. Put your hand on the valve and place your ear near to it. If you feel or hear any water running, it is a sign that the valve's seal is leaking. Most of the time, however, you will hear and feel a gentle "click" sound as the valve closes and water will cease running through the valve as the water is shut off.