From Water Wiki

Leaks ~ 14%

of Household Water Use

Leaks can be a huge source of water use in the house if they are not prevented or fixed. The most common sources of leaks are running toilets, dripping faucets and loose water connection to clothes and dish washers. Sometimes leaks are easy to pinpoint since we can see them when they are occurring, but others can cause water use without being apparent.

The best way to check for leaks in the house is to shut off all water in the house and check the meter to see if it indicates changes in water use over a two hour period. If the meter reading changes over the course of two hours even though you have shut off all indoor water then you probably have a leak. Newer meters are easier to use in determining leaks as they may have small indicator triangles that will move if you have all water shut off but have a leak in the house (h2ouse).
Some leaks may require you to call a plumber to have them fixed, but this is probably well worth it as even the smallest leak may be costing you over an extra 50$ a year on your water bill (cowlitz).

Sources:
www.ecobrownstone.com
http://www.cowlitz.org/naturalresources/water/leaks.pdf