Plumbers look for signs of water leaks in a variety of ways. One of the most obvious is a visible water spot that looks like a stain on a wall or ceiling.
Mold or mildew spots, along with musty odors
Peeling paint or wallpaper on the drywall or wood
Running water sounds when nothing is in use
Dripping faucets in sinks or outside on the hose bib
Sagging ceilings are very serious and could mean the leak has been there for a while
Damaged flooring that is warping slowly due to a leak
Reduced water pressure
Another method to detect a water leak is to check your water meter when no one is home. When you leave home, write down the meter reading and inspect the number when you return. If the meter number has moved at all, you likely have a water leak. Let a plumber know these changes in meter readings to determine how big of a leak it is.