For most water lines inside homes, the easiest way to find a leak is to inspect the entire plumbing system for visible water. For buried or otherwise inaccessible water lines however, finding the leak requires using a sensitive audio instrument. These sonic instruments require training to use properly, so finding an underground leak usually requires hiring a professional who knows how to use the equipment.
An easy way to test whether you have an underground leak or a leak inside your home is to put food coloring in the tanks of every toilet in the house and not use any other water fixture in your home for at least one hour. Check the water meter before starting the test and after the hour passes. If the meter did not move, then there is no leak either inside the home or underground. If the meter moved and colored water appears inside any toilet bowls, there is a leak inside the home. If the meter moved but no colored water traveled into any toilet bowls, the leak is underground.
Technicians locate underground leaks using a sensitive ground-penetrating microphone to listen for leaks. Once the technician hears a sound, she moves along the pipe until she finds the most intense sound source, which is the leak. After finding the leak, a plumber can excavate around the pipe and repair the pipe.