An experienced engineer can usually determine if there is a suspected leak in the plumbing system.  
    A leak in  the water line usually results in swelling of the soil and is seen as a heaving of the foundation that is localized. 

     A leak in the sewer system can result in one of two reactions: 
     1) If the leak allows water to enter the underlying soil, it can result is swelling of the soil, similar to the reaction seen with a leak in the water system, or
     2) if the leak  is sufficient as to allow soil to be transported down the sewer line, thus removing it from the foundaiton support system, a void can result under the slab.  In extreme cases this will result in settling of the interior of the building when the slab bends down because of the lack of support.


      In order to check the integrety of the sewer system, a cleanout has to be in place, preferably a double cleanout.