How do I find a water pipe buried under a concrete walkway?

I have a project the needs to cut into in house's main water line from the city.

What I found was that the main line from the city comes from the street and runs under a concrete walkway along the side of the house. I presume that the walkway is a typical 4" slab that was laid after the house was built.

A local plumber came out and suggested the best place for him to cut into the main water line was next were is pases next to the garage. But that means finding the main line under the walkway, cutting the concrete and digging so he can cut into the main line and reroute the new lines into the garage and back out.

How do I find the line under the concrete before I start?

Metal detector? Microphones? GPR? Tree branch?

Note: The house is built on a slab and all the home plumbing is in the slab. Other than re-piping the house, there is no good place we could find to branch into the main water line once it gets to the pressure reducer.