Use food coloring to find a toilet leak
Added
EPA WaterSense: put a few drops of food coloring in the toilet tank and wait 10 minutes without flushing. Color in the bowl means a leak.
The result
Result: EPA WaterSense: color in the bowl after 10 minutes means a leak; flappers should be checked periodically and replaced at least every five years. Flush right after the test to avoid staining.
Source
EPA WaterSense: Fix a Leak Week (opens the original source in a new tab). Check it for the latest numbers before you act.