A plumber typically starts with a pressure test to confirm a leak and judge its size, then narrows the location with acoustic listening equipment, thermal imaging, or a moisture meter. The goal is to open only the section of wall, ceiling, or floor where the pipe is actually failing.
On a well system, the plumber may also check the pressure tank, the switch, and the buried line from the well to the house, since a leak outside can look the same as one inside. If the leak is on the service line from the street, the plumber should help you sort out what the Water Division handles before digging.
Pipe replacement or repiping needs a permit from the plumbing and gas inspector in the Building Department, filed through the town's online SmartGov portal. A short spot repair may be done the day of the visit, while a whole-house repipe is usually planned and scheduled separately.