The signs, in the order people usually notice them
A water bill that jumps without the household changing. This is the most common way an underground leak gets found, and it is often the only sign for weeks.
The sound of running water when every fixture in the house is off. Stand still in a quiet room and listen. People dismiss this one because it seems too obvious to be real.
A warm spot on a tile or vinyl floor. That is a hot-water line under the slab, and it is the sign that most often gets mistaken for something else.
A patch of lawn that is greener, softer or growing faster than everything around it, especially during a dry stretch. Outside the house, that is usually where the line is.
Low pressure everywhere at once, cracks appearing in a slab or a driveway, or a musty smell with no visible source.
Confirm it with your own meter
This takes about an hour and it is the single most useful thing you can do before calling anyone, because it turns a suspicion into a fact.
Turn off every fixture and appliance that uses water. Ice makers and irrigation timers catch people out here — both will run on their own and ruin the test.
Find the water meter, usually in a box near the street. Note the reading, including the small sweep hand or the leak indicator if it has one. Many meters have a small triangle or dial that spins at the slightest flow.
Wait an hour with nothing running, then read it again. If the numbers moved, water is going somewhere.
To narrow it down, shut the valve where the supply enters the house and repeat. If the meter still moves, the leak is between the meter and the house — the yard. If it stops, the leak is inside, which on a Florida house usually means under the slab.
An hour with the meter turns "I think we have a leak" into "the leak is between the meter and the house". That is the difference between a guess and a starting point.
Why slab leaks are their own problem here
Most Florida homes sit on a concrete slab with the supply lines run underneath it. When one of those fails there is no crawl space to look in and nothing to see until the water has travelled.
Water under a slab moves along the path of least resistance, which means the wet spot inside the house is frequently nowhere near the actual break. That is why guessing and cutting is an expensive way to find it.
Older copper is the usual culprit, and the failure is often pinhole corrosion rather than a burst — a slow weep that runs for months. Polybutylene, which went into a lot of Florida homes before the mid-nineties, fails differently and is worth identifying if you have it.
What a professional does that you cannot
Acoustic listening equipment picks up the sound of pressurised water escaping through concrete, which is inaudible to an ear on the floor. Combined with a pressure test it locates the break rather than the symptom.
Thermal imaging finds a hot-water leak by the heat plume it leaves in the slab. Tracer gas is used where acoustics will not work.
The point of all of it is to open the floor in one place instead of several. Locating properly costs less than the concrete work it avoids.
What to do while you wait
If the meter confirms a leak and you can reach the main shutoff, turn it off. You can turn it back on when you need water. A slab leak is doing damage continuously, and the shutoff is free.
Photograph anything visible — the meter reading, the wet area, the warm patch, the bill. If this turns into an insurance conversation, dated photographs taken before anyone touched it are worth having.
Do not start breaking concrete to find it yourself. The wet spot is usually not the break.