How do I locate the leak in a pipe that is under a concrete slab?

Is this a drain or a supply pipe that you suspect is broken? Adrain can be viewed with a SeeSnake rodding machine. It will tell you the exact length from insertion and you can see/watch each turn and lay it out on the floor.

Bal gave you information about a drain pipe leak. However, most leaks are pressure lealks. They are hard to locate because water, like electricity, will seek the path of least resistance. This means that the hot spot you feel in the kitchen floor might be from a hot water leak 15 foot away. Rather then tear up a slab, and all our buildings are on slabs on the Gulf Coast, we call in a company that has equipment to locate and not only pinpoint it but break the cement and dig it up so the plumber can repair it. One such company is called Sleuth. I suggest that you call such a company to locate the leak.
Good luck, Tom