Record the meter reading on your water meter when no one is going to be in the home/building for at least 4 hours and no water is being used by dishwashers, washing machines, water softeners, etc. When you return, read the meter again. If the meter reading has changed, you have water leaking somewhere.
Put 10-12 drops of food coloring in your toilet tank; do not flush the toilet. Let stand for at least 1 hour. If the food coloring goes into the bowl, your toilet is leaking.
How often does your water softener regenerate? Some water softeners use large amounts of water in the regeneration process.