A "running" toilet can waste two gallons of water per minute. A silent leak can waste up to 7,000 gallons of water per month.
To find silent leaks, put food coloring in the tank. Do not flush. Check the toilet bowl ten minutes later. If you see color in the bowl, the tank has a silent leak.
Toilet leaks are typically caused by a worn out flapper valve or a fill valve that doesn't completely shut off when the tank is full. The flapper valve is the stopper in the bottom of the tank that lifts up when you push the flush handle and is the most likely culprit. The solution: replace your flapper. Shut off the water at the toilet, not at the house line. Remove the worn flapper and replace with a comparable replacement flapper valve (you can ask your hardware retailer for assistance in choosing the proper replacement.)