Leak Detection

Check the Pressure Relief Valve on the hot water tank. Sometimes these valves are plumbed directly into a drain and may be leaking without your knowledge. If you can't remove the drain pipe to check for a leak listen for a hissing sound, it may be leaking. 

Check the toilet for leaks by removing the top off the tank and listening very closely. If you hear any hissing at all, try to locate where it is coming from. If you locate the area where the leak is coming from, assess it and determine if you can fix it. If you can't, then call a plumber.

1. If nothing is noticeable, add some food coloring and put a couple of drops in the tank (not the bowl). Wait several minutes and if you have coloring in the bowl, you have a leak in the flapper at the bottom of the tank that is allowing water to seep through. At this point you can assess if you want to do the repair yourself, or call a plumber.
2. If you have more toilets, go ahead and repeat the process with each toilet to make sure you don't have more than one problem.

If the toilets are fine, check the line running from the meter to the house. While this may sound difficult, you can save money if you can locate the leak for the plumber.

1. If you know you have a shut-off valve by the house, shut it off temporarily and check the meter by removing the lid and watching the dial on top of the meter.
2. If you can't see the meter head, try digging around because they sometimes have dirt or grass covering the top of them. Once you locate it and the valve is turned off by the house, watch the meter to see if is turning. If it is still turning, then the leak is between the meter and the house. That is, unless you have a leaking valve, and this very common with these older bronze gate valves. Then, your leak may also be inside the house.
3. At this point, walk the area between the meter and the shut-off valve. Look for signs of a leak such as: soft muddy areas, grass that is greener than the rest or growing much faster than other areas. If you see such an obvious sign, call the plumber or assess if you can make a repair yourself.
4. If you have the valve shut off at the house and the meter has stopped moving, then the leak is somewhere in the house. Try some other techniques to try to locate the problem
5. Try and locate a leak by the house. This will require you to locate all the hose-bibs (hose-bibs are the pipes that you hook your hoses to, in case you were unsure!). Usually an average residence has one hose-bib in the front and one in the back, but be sure to find every one that you have and listen carefully.
6. Once you have located them, take a screwdriver, preferably one long enough to give yourself room to work, and put the metal tip of the screwdriver directly on the metal part of the hose-bib. Put your thumb knuckle on the top of the screwdriver, and then place your knuckle on the side of your head, immediately in front of your ear. The sound will travel directly to your ear drum. The idea, here, is for the solid screwdriver to work like a stethoscope. This works for most metal valves, as well.
7. Listen carefully for any sound emitting from the hose-bib. If you hear anything at all, remember where it is (perhaps mark it with chalk), and go to the next one. If the sound emitted gets louder at any of the other hose-bibs, then the leak is closer to that particular unit. Note that and contact your plumber: Giving the plumber this information will save the plumber loads of time in finding the leak, which in turn saves you money.
8. If you survey all the hose-bibs and still find no sound, go into the house and follow the same process with the screwdriver on your house fittings such as faucets in sinks, shower valves, washer, hot water heater (be careful to avoid being scalded when working around the hot water heater). If you are still not sure, just contact the plumber.

Check the shower head for leaks. It should be a fairly straightforward home repair if this is a source of leaking.

Check the garden. Look at hoses, taps, and drip irrigation systems

Recognize that in many cases a leak can be very hard to locate. Not all of the leaks outlined in this article can be located and if you're not used to plumbing positioning, you may miss something easily. All the same, if you try these steps, you should be able to find an approximate location and this is a most valuable exercise in itself because it will help the plumber (many plumbers do not like searching for a problem so anything you can do they will appreciate), making it time saving for the plumber and that translates into savings for you.