Start with a jack hammer. Maybe if you give more details we can too.

Is this a drainage or a pressure leak? Have you checked for a hidden leak? To check to see if you have a hidden leak first shut down everything in the house, no flush, no drinks of water, and if you are on a meter go out and check the little pointer in the gage. It should not move or creep. If it does you have a leak. If you're on a pump then check the pressure gage after the pump builds up to pressure and shuts off. The gage should not fall and the pump come back on. If so then you have a leak.
If you find that you have one a leak detection service called in to pinpoint the location of the leak. The slab will have to be opened up and the pipe repaired or replaced. Good luck, Tom

To repair a pressure leak beneath the slab, a jackhammer, hammer drill or sledge hammer if the labor is unlucky. Other options can include re-routes or new lines can be run depending on the circumstances. On a pressure leak, once you have checked the leak indicator on the shut-off valve as Tom suggests, try turning off the shut off valve in your home where the service line first comes above the slab, often in the laundry room, garage or front closet. If the leak indicator is spinning with this valve closed, the leak is on the service line, and may or may not actually be beneath the slab of your home. If the leak is on a drainage line, rather than a pressure one, a camera can be run to locate the leak so that is can be repaired.