Your water meter is the best detective in the home. It can tell whether you have sizeable leaks, as well as how much water various appliances are using.
Most meters record gallons just as your car's odometer records mileage. To find how much water you've used in any given period, just subtract the highest meter reading on your last bill from the current meter reading.
To detect a leak, turn off all water in your house and observe the meter. If the red triangle dial is moving at all, water is leaking somewhere since this dial will record even the smallest of leaks. If the red triangular dial is moving rapidly, it probably indicates a major leak, either from the line that goes from the meter to the house or inside the house. With a major leak, the black numbers on your meter will turn also.
The toilet is one of the most common - certainly one of the sneakiest - and its leaks tend to be invisible. Most of them occur at the overflow pipe or at the plunger ball inside the tank. If you've never watched what happens in there, take the tank lid off, flush and pay attention. The water level should come up to about a half inch or so below the overflow pipe. Gently bend the float arm down, if necessary, so the valve shuts off the water at that level. If the valve itself is leaking, you may need a plumber. That's a bit trickier to fix.*
Finally, drop a little food coloring into the tank and - without flushing - see if it comes out in the bowl. If so, you probably have a leak in or around the plunger ball down at the bottom of the tank.
*American Water Works Association,
"Be a Leak Seeker"
*Source: American Water Works Association,
"The Story of Drinking Water"