I have a water leak about 1 1/2 inch in diameter in the ceiling of the living room. The upstairs bathroom is above the leak. How can I find out if it is coming from the shower or the toilet? Or does it matter? Does the water from both drain through the same pipe?

You going to have to patch the ceiling anyway and wet sheet rock is great food for mold. Remove some ceiling and have a look with a flashlight. Water leaks are not un to find. Water runs along things and drips sometimes not even near the leak.

KISS is right about that I have had some very frustrating leaks to track down anyway as far as yours goes I agree with KISS tear some ceiling out and look around in my experience most leaks come from drains not so much from the actual drain pipes although it does happen on rare occasion. When you look have someone else run a little water down the shower drain and maybe flush the toilet a couple of times and just see what you see. With any luck you will spot it right off . Post back and let us know what you find. GOOD LUCK