To find out where a plumbing leak is coming from, start by turning off the main water supply valve to prevent further damage. Then, inspect the area around the leak for any visible signs of water damage, such as dampness or mold growth. Listen for any hissing or dripping sounds that may indicate the location of the leak. Check the water meter to see if it's still running, which could indicate a hidden leak. Finally, consider hiring a licensed and experienced plumber to use specialized tools, such as infrared cameras or moisture meters, to accurately locate and repair the leak.