To locate the source of a water line leak, start by checking for signs of water damage, such as wet spots, mold, or discoloration, both indoors and outdoors. Turn off all water fixtures and monitor your water meter; if it continues to register usage, there may be a leak. Professional plumbers often use specialized equipment like acoustic leak detection devices or thermal imaging cameras to pinpoint the exact location of underground leaks. Consulting a licensed plumber can help expedite the detection process.