How would I find a punctured hot water line in a cement garage floor heating system?

The hot water baseboard system to the house was powered by a coal boiler, but now is converted to electric, with the coal as backup, if needed. The garage floor has never been heated by the heated water pipes as it was punctured early in the pouring of the cement, many years ago. Before we go to the expense of adding a heater to the garage, I'd like to explore ways of finding the location of the leak and then cutting into the cement and fixing it. (The water source has been shut off, of course.) Would a mechanic's stethoscope work once the water was running?
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