Use Food Coloring
To find the source of a toilet leak, put food coloring in the tank. Colored water in the bowl means the flapper (most times is a rubber seal at the bottom of the tank that’s connected to the flush level rod by a chain inside the tank that holds and releases water into the bowl when flushing occurs) is faulty. If it drips down the outside, replace the seal between the tank and bowl. Colored water on the floor indicates a cracked and/or damaged wax seal beneath the toilet.