If your toilet is not the culprit, other leaks are not so easy to see and can also cause your water bill to be high.
If you have a high water bill and the toilet seems to be operating properly:
Check all faucets. Replace worn washers or defective fixtures.
Check outside water taps to be sure they are turned off at the faucet.
Check your water heater overflow pipe.
Check a humidifier tank that has a miniature float arm and an overflow.
Check for an underground leak.
If these steps do not solve your problem, you may need to call a plumber.
Toiletology: http://www.toiletology.com/index.shtml
