We were told by the water company that we have a slow leak - a plumber suggested we turn off the water going to the house and then check the gauge to see if the leak is in the house or in the ground. I know where the shut-off valve by the gauge is, but he said there is another one either in or just outside the house. Where would it be and what does it look like?
Is there a way to find the plumbing schematics of my house?

Locate the cold water main, (start at the water heater) and track it back to where the water service enters your house. There should be a shut off there.
The house shut off looks like a ordinary in line shut off valve.(See image)