To check for leaks:

Turn off all the sinks, dishwashers, ice makers, irrigation systems and other water-using devices in your home. 

Then, check the leak detector triangle on your water meter.  If it's spinning after everything is turned off, you probably have a leak somewhere!

If you do detect a leak, check to see if it's your toilet by adding a few drops of food coloring to the water inside the tank.  Wait a few minutes.  If the water inside the bowl changes colors, then you've got a leak.  Simply replacing the rubber flapper valve inside the tank can often fix leaks like these.