A water main leak can occur without the householder even knowing about it. There may be subtle signs of a leak, such as dripping water without a visible source or a higher water bill. Fortunately, there is an easy, foolproof method for homeowners to find out whether they do have a water main leak.

1. Locate the water meter

Start by finding out the location of the water meter. This is usually located in the exterior of the home. The water metre will be connected to the key supply pipe that connects the property, and it will usually be clearly labelled.

2. Record the number

Wait until nighttime, after everyone has taken their bath or shower and gone to bed. Check the water meter and write down the numbers on the meter.

3. Check the meter number

In the morning, wake before the rest of the household and before anyone uses any water-related appliances. Check the meter. Small changes can be explained by toilet use or family members drinking water during the night. If there is a jump in the number, there could be a water mains leak. In that case, contact a plumber for advice.

Source:http://plumbinghelptoday.com/denver-plumbing-repair-blog/2013/07/water-main-leak-check/