A water leak can waste hundreds of gallons of water. You'll also be paying for water that you aren't even using! Here is how to find out if you have a leak.
If you know you are currently not using any water, go and check the water meter. If the pinwheel is moving, that indicates water is currently going through the meter and you have a leak.
Find Leaks and Fix Them.
Put a few drops of food coloring in the reservoir. If colored water appears in the bowl after a few minutes, the flapper is leaking. Check the water level in the tank. Is it at or near the top of the overflow tube? If so, adjust float to a lower position (about an inch below the top of overflow tube is normal).
How often does your softener regenerate? Check the time clock for time of regeneration. If you hear your softener run at any other time, you have a problem. Check with your softener company or a plumber. Another option is to take readings of your water meter for a few days at the same time of day (e.g. every morning before you leave for work). Then bypass your softener and do the same thing. If you consistently use less water (keeping in mind that regeneration uses water; check your owner's manual for amount) in bypass, you may have a problem. Water constantly flowing from the drain tube is an indicator that the softener is stuck in a backwash or rinse cycle.
Even small drips can add hundreds of gallons of usage to your water bill. Remember it is dripping 24/7. Put a glass under the drip. You may be very surprised how quickly it fills up! Valve cartridges in the faucet handle(s) are often the culprit.
Check your time clock. How long does each zone run? Turn it on and the time zones. Does it match? It could be sicking valves or a problem in the programming. Periodically, walk your lawn, especially near the heads. Are there wet/spongy/overly damp areas? You may have a leaky head or pipe problem.
Does it seem to run constantly, even when the furnace isn't? Check the overflow tubing. Is it always damp around the end of the tube or is water running constantly - even at a slow rate? If so, you may have a problem.