Your water bill is going up
If your usage did not change but the bill did, a leak is the most common reason.
You hear running water
Water sounds when all taps are closed often mean a leak inside a wall or under the slab.
Mold or musty smell
Moisture from a slow leak feeds mold in cabinets and walls.
Warm spots on the floor
A hot water line leaking under the slab can make part of the floor warm.
Low water pressure
A sudden drop in pressure can be a sign of a leak in the supply line.