A tub spout leak is one of the most insidious of all plumbing leaks because the primary drip area is inside the wall, where it is difficult to spot until considerable damage has occurred. The spout threads corrode or crack, sending water along the pipe and down inside the wall.
One of the first signs of a tub spout leak is a reduction in water flow. The tub or shower might still have water flowing, but the amount and pressure are likely to be less than normal. As time goes by, a darker spot is likely to appear on sheet rock in the area around the tub or shower, although this takes some time as the closest walls are generally tile or fiberglass.
A sign that the leak has reached a severe point are water beneath the carpet in the bathroom around the tub or shower. With a tile floor, this damage would take more time to appear, as the water under the floor would have to erode the adhesive holding the tile in place. The best way to keep this type of damage from spreading is to remove the spout and take a close look at the threads. Any sign of oxidation or corrosion means that it is time to replace the spout. If the lower pressure persists after the new spout is in place, the leak may be somewhere else, and it may be time to call in a plumber.