Go through your home and make certain that all the faucets are turned off and that the toilets have indeed stopped filling the reserve. Then, go to the water meter and make a mark on the lens where the dial is currently. If there is water in the meter box, the meter itself could be leaking.
If the meter is still turning, the leak is between the meter and the house. By inspecting the area between the meter and the house, you can look for soft, muddy areas or grass that is greener than the rest of the yard.
One of the hardest places to isolate a leak is in a swimming pool. If you have an automatic filler, like in a toilet, you'll need to turn it off. Mark the water line on the wall and wait to see if the water level goes down. There will be a certain amount attributable to evaporation.
Some leaks can be very difficult to locate. Plumbers, by the very nature of their job, will be more familiar with tracking down the source of the leak than a homeowner. There are some non-invasive techniques like acoustic listening devices, heat scanners and miniature video cameras on fiber optics that professionals can use.
Leaks can be expensive from the loss of water and the resulting damage that it can cause. Determining where the location of the leak can also cause damage because plumbing is usually concealed in walls or under concrete. For particularly difficult to locate leaks, discuss how the professional intends to locate the leak and minimize damage in the process.