Plumbers find an underground water leak by narrowing it down with several tools rather than one. The usual stack is acoustic listening gear and leak noise correlators to hear water escaping the pipe, infrared cameras and moisture meters to spot the wet ground, and a pressure test to confirm which line is losing water.
Acoustic equipment does most of the work. Water forced out of a pressurized line makes a steady hiss, and a ground microphone or a sensor clamped to a valve picks that sound up through the soil. A correlator places two sensors on either side of the leak and uses the slight difference in when each one hears it to calculate the spot. On a suspected slab leak the same listening is done across the floor, so the point is located before any concrete is opened.
Other tools cover what sound misses. An infrared camera reads the temperature change a buried line leaves at the surface, a tracer gas can be sent through the pipe to rise up where it escapes, and isolating and pressure testing each line shows which system is failing. A good first check costs nothing: with every fixture off, a meter that still creeps means water is escaping on the line you own, the same sign as water standing in the meter box. For the yard-line version, see the underground leak detection guide.
No single tool finds every leak. Plastic and concrete pipe muffle the sound acoustic gear depends on, so readings are less certain than on metal, and a deep or well insulated line can hide from an infrared camera. That is why the methods are combined and picked to fit the pipe material, its depth, and how the ground or slab above it can be reached.