They just drive down the street very slowly and a digital reader picks up from inside the truck.
Our bill this month was our average winter bill, but make sure you don't have a leak by turning everything in the house off and checking to see if the meter is moving. We had a leak when we moved in that was undetectable any other way. The sprinkler company had incorrectly tapped in to the main line and we saw no evidence because it was washing out a huge sink hole under our sidewalk. Our first few months bills were all between $300-$400, then we finally got it figured out.