The easiest way to determine if you have a leak is to shut off all water-using appliances, faucets and outside watering tools (do not shut off the main water line to your home), and then check to see if the flow indicator on your water meter has completely stopped moving. If you’re sure no water is being used inside or outside the home, and the indicator continues to move, you've probably got a leak somewhere. In rare instances your water meter may be faulty, but it’s unlikely.
At that point, you will need to determine where the leak is coming from.
According to EPA, household leaks can waste 180 gallons per week, and toilets are often the culprit. Some leaking toilets make a noise while others might be visible (a slight trickle from the rim of the bowl to the water below). You may not be able to tell whether your toilet is leaking. To test whether a toilet has a leak, you can purchase toilet tank leak detector tablets, but it's simpler just to place a few drops of food coloring in the holding tank. If color appears in the bowl one hour later, your toilet is leaking.
Check all your faucets to see if they are dripping – be sure to check under the sink as well!
If it’s not your faucet or toilet, the next most likely culprit is your irrigation system. Look at your lawn. If you find wet spots or pools of water around your spray heads, you've probably found your irrigation system leak. Also, brown spots and boggy spots can help pinpoint problems. If you have a layout of your irrigation system, start by tracking the lines to recognize trouble spots.