There are several ways to tell if you have a leaking pipe underground. One way is to listen for the sound of running water when there should be none. Another way is to look for wet spots or puddles in your yard that weren't there before. Yet another way is to use a moisture meter or soil probe to check for moisture in the ground where the leak is suspected. If you suspect a leak but aren't sure, it's best to call a professional plumber to come and take a look.