Hold off on tearing into the ceiling. Although it will probably have to be repaired eventually, and it is true that opening it up may provide visual location of the actual leak, start by investigating the shower. Remove the little chrome eschtcheon around the shower arm ,and look in there with a flashlight. Remove the trim plate from the shower valve, and look back there with a flashlight, and lastly, if the shower is turned on by pulling up a button on the tub spout, then remove the spout and see if ther is evidence of a leak on that connection, going back into the wall. It can have a leak which does not occur during a bath, but only during a shower when pressure is increased at that tub connection.