You can do it yourself.
Before you call a plumber or a leak locating service, there are ways that you can confirm or eliminate the possibility of an underground leak. If you have a leak under the home there are some ways that you can know for yourself if you have a slab leak or not. You probably don't need a professional until you know you have an under-slab leak and you're ready for electronic leak locating services.
If you call a plumber or leak locating service, they will go through these steps for you but if you can do them yourself, you can save money!
The first thing you should do to begin your leak confirmation is to check your water meter. The water meter for a typical residence is in the front yard near the street or sidewalk. Remove the cover using a long screwdriver or pry-bar of some kind. The meter indicators are usually under a glass or plastic lens. Clean the lens so you can see the indicators inside.
Most water meters will have numbers that advance like those in a car that show mileage. When water is flowing hard to your home from the city, this meter may indicate movement. However, a smaller flow of water can be very subtle especially in the case of a pin-hole leak under the slab. The numbers indicator won't be much help so notice that there is a turning wheel of some sort which is also under the same lens. This tiny wheel is usually red and spins fast when water is flowing fast but slower when the flow is slower.
If there is any movement at all indicated at the water meter, it's time to do some checking inside and outside the home. You'll want to isolate all the sources of water usage and shut it down. Consider the following areas of your home and turn the water off to these areas if there is usage.
1) Check any fountains that have automatic floats. There will be a shut-off valve somewhere near the water fountain. Turn off the water there temporarily during this investigation.
2) Check your water heater to be sure it isn't leaking in any way.
3) Check your pool's automatic fill valve. If there is any hissing sounds here, this will be at least partially responsible for any movement you see from the water meter. There will be a valve nearby that can turn this valve off temporarily to eliminate it for now.
4) Do you have any miscellaneous fixtures such as a drinking fountain? Make sure they aren't leaking or being used during this test. If there is a leak, reach under the fixture and turn it off at the angle stop.
5) Consider the irrigation system including sprinklers. If the sprinklers aren't running, you are probably safe to assume that they aren't a source for water usage. However, be sure they aren't leaking. Check inside the valve box for moisture. If there is a leak here, just turn off the valve that leads to the irrigation box just temporarily.
6) Do you have a swamp cooler or evaporative cooler? Check to be sure that the switch for the evap cooler is turned off. If there is a leak of any kind, find the supply valve and turn it off.
7) Check your water softener that it isn't cycling or leaking. If in doubt, turn off the water softner supply valve which should be above the unit or nearby.
8) Look under the kitchen sink and be sure there are no leaks from your RO system.
9) Trap primers supply a continuous drip to floor drains and should be eliminated as a potential water usage source. Trap primers are commonly located at the back of a cabinet near a floor drain.
10) Check all faucets for drips or leaks.
11) Do you have an ice maker either dedicated or integrated with your freezer? Make sure it isn't making ice by turning the supply valve off. If this is too much trouble because of access problems, leave it be for now but consider it as a last resort.
12) Make sure your clothes washing machine isn't running or leaking. If it's turned off, look at the valves in the back of the wall. Is is leaking there? Turn the valves off then unscrew both hoses and check to see if there are any leaks coming from the washing machine bibs.
13) Check all your bathroom fixtures such as urinals, toilets, faucets, tub spouts and shower valves. Are they dripping? If so try to turn off leaking fixtures. Faucets will have stop valves inside the cabinet or vanity where both the hot and cold side can be turned off. A leaking tub or shower faucet is a little more difficult since there are no isolating valves.
Now, after checking all that, go back to the water meter? Is it still moving? If so, check your floor for hot spots or water spots.
At this point, you've either found the source of water usage or you need a professional to go to the next step. You more than likely need to call a plumber with electronic pipe locating equipment and expertise in leak locating and slab leak repair. A qualified plumber can locate underground leaks and leaks under concrete floors. Slab leak repair requires saw-cutting or jackhammering. A qualified plumbing company will have all the necessary equipment and expertise to do a slab leak repair.