Checking for a leaky toilet is easy and just takes a few minutes. Leaky toilets can waste thousands of gallons of water each year, if left unchecked. This test is an easy way to expose even the slowest of leaks.
Here is how it is done:

Step 1: Pull the cover off the back toilet tank.

Photo: mywaterfuture

Step 2: Drop a bit of food coloring into the water. Don’t worry, it won’t hurt anything inside the tank.

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Photo: mywaterfuture

Step 3: Wait 15 minutes. If you see any of the dye, you have a leaky flapper.

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Photo: mywaterfuture

(*Note: Does your toilet only leak at night? Many toilets really do run only at night; that’s because people use much less water during these hours and in many municipal water systems, the water pressure rises considerably during this time. It is not uncommon for system water pressure to rise by 15 to 20 pounds per square inch (psi), and by as much as 30 psi in some situations. This rise in pressure could cause “water creep” inside your gravity-fed tank by 1/2 inch or more.)
Quick fix: Adjust the float in your toilet tank so that the water level is lower. This will prevent the added water pressure from causing water overflow into the bowl.