High water bills? Wet patches in your yard? Strange noise coming from pipework?
Check your water meter before you go to work or before you go to bed at night, record the meter reading.
When you return from work or get up in the morning (provided you haven’t used any water) the second reading should still be the same as the first.
If the two readings are not the same there is a good chance you have a water leak.
Common causes are dripping taps, running toilets, a burst hot water system or burst pipe.