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.
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.
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.