Signs of a leaking drain pipe include water stains on walls or ceilings, a musty smell, bubbling paint, sagging floors, uneven tiles, and the presence of rats or mice. These signs could indicate that water is seeping through walls or ceilings, causing paint to bubble, weakening floors, or making tiles loose and uneven. Rats and mice may also be attracted to the water and living in the walls or ceilings near the leak.