A leaking tub plug can be difficult to remedy because there are various possible causes and most of them can be behind drywall.
1. Inspecting the Shower
To ensure that it is really your plug that is leaking make sure that the water is not just draining off the tub corner while there is someone in the shower. Inspect the walls and flooring around the tub as soon as someone is done showering. Take a closer look at the side where the shower head is located. Many supposed tub leaks are caused by water that's running off its edge.
2. Checking your Connections
If your bath tub leak is constant, even when no one has been in it, the leak must be on your water line. These are subjected to constant water pressure and are prone to leakages. If the leak is not constant, you can rule out your hot and cold water lines and start looking somewhere else.
3. Testing Your Tub
In testing your tub, do not use the faucet. Instead fill a bucket from another source and fill your tub. Once you have it filled, let the water stand for around 30 minutes and see if you can see the leak. It you do, it is coming from the tub drain. If it does not, drain the tub and check for leaks again. If it shows up this time, it is coming from the drain pipe.