The warning signs
A water bill that's crept up with no change in usage, damp or discoloured patches on walls and ceilings, a musty smell, warm spots on the floor from a leaking hot pipe, or a boiler that keeps losing pressure. Any of those is worth a closer look.
A quick test you can do
Turn everything off, note the reading on your water meter, then wait an hour without using any water and check again. If the reading's moved, water's escaping somewhere it shouldn't be.
Then what
The trick is narrowing it down before opening anything up, so you're not pulling up half a floor on a hunch. That's the bit we're good at, tracing it with as little disruption as possible, then repairing it properly.
Where they usually hide
Hidden leaks tend to turn up in the same handful of places: under a kitchen or bathroom floor, behind a tiled wall, on the heating pipes buried in the screed, or out on the supply pipe between the boundary and the house. Because the water tracks along joists and pipework before it shows, the damp patch is rarely right over the leak, which is exactly why guessing and digging in is the expensive way to do it.