Step 1. Remove the toilet tank lid and add a small amount of red food coloring to the tank’s water.
Step 2. Replace toilet tank lid and do not use toilet for at least an hour.
Step 3. After an hour, check to see if the water in your toilet bowl has any signs of the red dye. If it does, then the water in the tank is continuously leaking into the toilet bowl. Be sure to flush and clean your toilet immediately following the test to prevent dye stains.
Most toilet tank leaks can be fixed by replacing your tanks internal parts. To test for a hairline crack in the toilet bowl, use the same procedure but put the dye directly into the toilet bowl.