A quick place to start is with the toilets in the home. Toilet leaks might be the most invisible leaks of all, since all the evidence goes into a bowl of other water, then down the drain. Toilets can leak from tank to bowl or bowl to drain without your ever being aware of it. To confirm your toilet is leaking water from its tank to the bowl between flushes, simply put some dye in the tank. Food coloring works fine but must be flushed right after the test to avoid staining.) If the toilet remains unused and you have a leak, you will see color in the bowl within about 20 minutes.
When you have a toilet leaking water from the tank to the bowl, it's almost always caused by one of a few mechanical components inside the tank. A worn, warped, or disintegrating flapper that doesn't form a good seal between the tank and bowl is the most likely culprit if your toilet fails the dye test. These are usually very simple DIY replacements for which you can find a kit at your local hardware store. Dye in the bowl can also be caused by a toilet's handle mechanisms, which are easy to replace. A malfunctioning fill valve can cause water to enter the bowl via its overflow tube. Replacement fill valves look more daunting than they are. In all cases, these can be replaced by simply removing the old devices, sticking in the replacement, and tightening everything back down.
If you suspect a leak, monitoring your home’s water meter will give you a definitive answer. The meter is often located beneath a manhole-type cover in your yard. Most of the time there is a white post that is about 5 ft tall sticking up from the ground near the manhole.