Leaks from a bathtub can be frustrating to diagnose. There are several different possible sources for leaks at tubs and most of them are behind drywall. A leak could be from the valve, the shower head, the tub spout or the drain.
Since each of these possible locations are different, it would be nice to isolate the problem. Before you start cutting drywall there are a few things you can do to try to narrow your choices. Then, even if you do have to cut drywall you will usually have to do less damage.
It Might Not Be A Leak
First, make very sure that the water is not just running down the corner of the tub past the shower curtain while your someone is taking a shower. Take a very close look at the wall and floor corners at both ends of the tub immediately after they are finished. Pay special attention to the end the shower head is on. I have seen many "leaks" that wound up being caused by water running over the edge of the tub. Enough people have this problem that you can but special little dams that stick in the corner on the tub to block the water.
If you are sure this is not the problem the next step is to determine whether the leak is from the valve or from the drain.
Water Lines or Drain?
If the leak is constant, even when the tub has not been used, it is a leak on the hot or cold water line. These are under constant pressure and will leak continuously if they are the problem.
If it does not leak all the time, it's not from the hot or cold connections. If it only leaks after use you can rule these two connections out and start elsewhere ..
The Bucket Test
The easiest thing to check first is the drain side. You do not want to run the faucet because that will screw up this test. Fill a bucket up from another bathroom and use this to fill the tub. I know that's a pain in the neck but it's necessary.
Once the tub is full let it sit for 30 minutes or so and see if the leak shows up. If so, it is from the tub drain. Drain the water and fix the drain. It probably just needs to be removed and re puttied. Clear silicone, though harder to clean up, works great instead of putty.
Pull The Plug
If no leak shows up while the tub is full go ahead and drain the tub, again checking for leaks. If you see a leak now the problem is in the drain piping. You will need to cut drywall to find and repair this. You can start at the wet spot, but you may find that the water has run across the top of the drywall and the spot is not directly under the leak.
The Water Piping
If no leak shows up from the first test, that leaves the tub spout piping and the shower head riser and connection. The two most likely places to check for a leak are the shower riser and connection and the tub spout piping and connection.
The first thing to do is to pull the escutcheon (trim ring) at the shower head and see if you can see the connection inside the wall. You may have to open the hole a little, just enough so you can see but it will still be covered by the escutcheon. Now run the shower head while looking at the connection with a flashlight.
The Old Toilet Paper Trick
If you can not see it very good, wrap some toilet paper around the end of a screwdriver and hold it under the connection for a minute. Check and see if the paper is wet. If so, or if you can see a leak, you've found the problem. Take the shower head off, stick the handle of your pliers in the end of the bent pipe coming out of the wall and remove the pipe. Put some new teflon tape on the threads, screw it back in and check for leaks.
If you do not find a leak here check the tub spout. If you have the kind of tub spout with a diverter you have to pull to make the water go to the shower head it could be leaking where it connects to the piping and water could be running back into the wall. This is kind of hard to see but some brands have a small opening on the bottom side of the spout to tighten the spout clamp. You can use the toilet paper trick here too. If there is no opening you can try to unscrew the spout and see if the piping behind it is wet. If none of this works you may need to cut drywall behind the tub to really check it out (if you can get to it).
The Last Resort
If you try all of this and still can not find the leak the next step is to start cutting drywall. However, if you can find the source of the leak this way you may be able to fix it yourself and save some money. Even if you do not feel comfortable doing this, you will at least be able to let the plumber know what you've done and minimize the cost of the repair.