If you have a leak under your foundation, this usually comes from a water line or sewer line. This could be a slab leak, which is a hidden leak. And the following 4 signs may help you determine if you have this type of leak. 1. Watch Your Water Bills The first thing you should start monitoring is your water bill. Save all your bills, and follow your regular water expenses. You can easily detect a leak when your bill is higher than it normally is.