If a slab leak is suspected there are a few different methods used to confirm it:

Visual inspection: sometimes you can tell just from looking at the water meter.  You may see wet planters around your home and water inside your home near the base boards, for example.

Thermograph inspection: locates the leak by comparing the temperature of the affected area to the surrounding area.  Cameras that can detect Infrared radiation are used to find the areas that are warmer or cooler, which may be the location of water loss.  It can also be used to identify water in locations that are hard to check, such as inside walls and the roof.

Ultrasound inspection: uses sound waves to identify the location of the leak.  This can be done in two ways: 1) by identifying the sound of water coming out of the pipe, and 2) by passing air through the line and then locating the sound made by the air as it escapes from the line.

Helium detection: helium is passed through the lines and escapes through the leak.  A tool is used to find the place where the helium is escaping.  Helium molecules are small enough to fit through even the tiniest holes, and it does not react with whatever is in the pipe so this method is useful when adding liquid to a pipe is not desirable.

These can be used separately or together to help identify the leak.

The alternative is to just start digging and tearing up the ground and hoping you find it, and is often done by unskilled workers. That’s not the way you want.