Just one leaky faucet can waste as much as 20 gallons of water a day and make a significant impact on a water bill. Start by checking all indoor faucets and outdoors spigots for obvious drips and leaks. If you have a lawn irrigation system, you should also check for broken sprinkler heads, breaks in pipes or tubing, or missing emitters. If that is not the problem, you will need to check for less obvious leaks.
Shut off all water fixtures in your home, including any water-using appliances. Add a dye tablet or a few drops of food coloring to each toilet tank, and read your water meter. Do not use any water or flush the toilets for at least an hour. Recheck your meter reading and the water in your toilet bowls. If the water in a toilet bowl has changed colors, you have a leak in that toilet. If the water in your toilets is still clear, but your water meter has moved, there is a good chance that your leak is underground. A plumbing expert or leak detection service can help you identify the source of an underground leak.