It's a fair question — and the honest answer is a combination of listening equipment, thermal imaging, and process of elimination.
Specialized microphones and amplifiers detect the specific sound of pressurized water escaping a pipe, even through drywall or concrete.
These reveal temperature differences in walls and floors that often correspond to moisture or hot water leaks, invisible to the naked eye.
Testing different sections of the plumbing system in isolation narrows down which line and zone is affected before any physical search even begins.
No single method is perfect on its own, which is why combining them narrows the leak down to a small, specific, confirmable area.
Let's get you a real answer, not more guessing.