Damp patch?
If you have a damp patch in the garden or one area of the lawn that that is always green, you could have a water leak.
Hear running water?
Can you hear water running into a drain when no water is being used at your property or can you hear hissing? This might suggest a water leak.
Low water pressure?
Low water pressure, high meter readings and bills all suggest a water leak.
Water meter readings
Check the dials on your water metre. If they are moving even when you are not using water or if there is a significant difference between night and morning, you could have a water leak.