Check to see if your water meter has a "low flow indicator" on it. A low flow indicator is very useful in problem solving if you suspect you have a leak in your home or business. When you are sure that nobody is using water, check the black arrow or red dial, depending on which type of meter you have. It should not be turning. The slightest movement means that water is being registered by the meter. A meter will not turn unless water is flowing through it.
If the low flow indicator is turning, you will need to start looking for the source of the leak. Start with the toilet. You can turn the valve off at the toilet, then check the low flow indicator on the water meter. Another way to check a toilet for a leak is by placing a dye tablet or food coloring in the tank. Let it sit for a while, and then look in the bowl. If you see coloring, the toilet is leaking.
Continue isolating areas until you have narrowed the leak down.