There are telltale signs. A water bill that has gone up for no reason, the sound of running water with the fixtures off, or a pressure drop are all common. You might also notice damp floors, warm spots (which can point to a hot-water pipe near the heater), mildew smells, or even cracks in the floor and moisture at the baseboards. Water can show up around the concrete slab as well.
It is not wise to put off dealing with a suspected plumbing problem. Left to it, water under the concrete will do considerable harm to the finishes near the foundation and set the stage for mold. An experienced plumber will be able to test the system and find the source without having to open up the floor first.