There is water leaking from my upstairs tub, coming out in the ceiling downstairs. It is not the drain line, as it only leaks after shower has been on a few minutes. The leak is coming out downstairs at a spot at the back of the upstairs tub... I suspect the water is leaking from the shower line and running towards the end. What can I do?

Unscrew shower head. Plug it with 1/2" galvanized plug and turn water ON. If it starts leaking from the ceiling again, you know you have just isolated the problem. Leak is in the shower riser.

You may also have leak from shower pan. Plug shower pan, fill it with water. See if it leaks from the ceiling.

Best would be if you can cut open ceiling at the point of the leak. Stick your head inside and inspect P-trap and shower drain. At this point you can also see if the water is coming from the shower riser.