You can verify if your toilet is leaking by dropping a dye tablet in the tank. Food coloring works too. Then wait 15 to 20 minutes and check the bowl. If there’s colored water inside, there is a leak. Another way to check is to look in the tank. If the water level is higher than the overflow pipe, the cause may be a fill valve problem. A third test is to turn the water off overnight to the suspect toilet and check if the water level has fallen to less than an inch by morning. Also, an unused toilet that’s making a running noise or has water dripping down inside the bowl is leaking.