Water can leak out of pipes in a bathroom before you are aware of the problem, dripping onto the ceiling in the room below. Prolonged exposure of water to drywall may make replacing the drywall ceiling necessary, as well as causing problems with the electrical wiring or lights in the room below. Finding the source of a plumbing leak takes some careful examination, even if you think you already know the leak's location.
Examine the ceiling in the room below the bathroom, if one exists, looking for any discolourations on the ceiling or the presence of water. While the presence of water may indicate a leak coming from the bathroom's plumbing, the location of the water on the ceiling may not indicate where the leak is since water may run along a pipe before dripping.
Run the bathtub's faucet and keep the stopper on the tub's drain open, allowing the water to drain freely for 15 minutes or more. Examine the ceiling in the room below the bathroom, looking for signs of leaking water.
Place a large bucket or other container in the bathtub, directly below the tub's spout. Turn both the hot and cold taps on. If water collects on the ceiling below the bathroom, the leak is coming from the faucet connections in the wall and not a pipe under the bathtub.