Follow these simple instructions to find out if a leaky toilet is robbing you of precious gallons and dollars. You can detect a leak by removing the toilet tank lid and placing leak detection dye tablets, a few drops of food coloring, or some laundry-bluing agent in the toilet tank.
Do not flush the toilet. If you see color in the toilet bowl after 15 minutes, you have a leak. Two common toilet leak locations are at the overflow pipe or the flapper valve. To determine if the leak is at the overflow pipe, look for water flowing over the top of the overflow pipe. If it is overflowing, the water level is too high.
If you have a toilet manufactured before 1982 that uses 5-7 gallons per flush, you can adjust the water level by gently bending the float arm down so that the valve shuts off when the water level is a half-inch below the top of the overflow pipe. Many 3.5-gallon toilets (manufactured after 1982) can also be adjusted the same way.
Some 3.5-gallon toilets, or ultra low flow toilets that use 1.6 gallons or less per flush, may have an adjustment screw on the float arm to lower the water level.  Sometimes the ballcock assembly itself is worn, causing water to run continuously into the tank. If the ballcock assembly is worn, it needs to be replaced.
If the leak is not at the overflow pipe, check the flapper valve. If it appears to be deteriorated or does not seal completely, it should be replaced. You can find a replacement flapper valve at your local hardware store.
If your toilet still leaks, you may need the assistance of a plumber or handyman.