A water leak inside a wall can be caused by malfunctioning plumbing or a roof that has been compromised. Finding the source of a leak can be difficult because the place where you see the water may not be where it's actually leaking. Water can run horizontally along pipes inside the wall before it finds its way out. You can mitigate the amount of wall repair that will have to be done after the leak is fixed.
Observe the conditions under which the leak occurs. If the leak happens when it rains, check the roof for a damaged area where water may be entering.
Turn on the water in a sink, shower, or tub above the leak to determine if the source is a bad drain. If the leak seems to occur only when water is running, then a drain pipe may be the cause. If the home has drop ceilings, you can remove some tiles and find the leaky drain pipe. If the home has a basement, find the source of the leak from there.
Place your ear against the wall near the leak and listen closely for the sound of dripping water. The dripping sound may be louder the closer to the source of the leak you are. The more you can narrow down where the leak is, the less damage to the wall you will do to fix it.
Make a slit in the wall with the utility knife and use the drywall saw to cut out a square that's large enough to put your head inside where you think the leak is located. Once you can see inside the wall, use a flashlight to find the source of the leak. If necessary, cut a hole in another location in the wall once you're able to see inside the wall.