A plan of attack
Don't despair yet -- you're on the right track. As Bob said, your plumbing likely has no joints under the slab. The lines loop up at each fixture location and are joined above the slab to allow connecting the fixture and continuing the line on to the next fixture. If there's only one line going into the slab at the boiler, it's a linear array down the line. Some more complicated configurations exist, but are rarely found unless it's custom construction.
The next task is to figure out what sequence the fixtures are in, and then determine which under-slab link the failure occurs in. The first task is the harder of the two, but not impossible. It's all easier than it sounds -- sort of -- especially on the hot-water side. We'll do the second part first.
With all the above-slab junctions exposed, just turn off the hot water at the boiler and go to bed (I said this would be easy). First thing in the morning, turn on the hot water again, have a cup of coffee, and relax for a few minutes to allow some hot water to flow from the boiler to the leak -- but DON'T draw any hot water from any fixture. Then go feel every hot-water junction. If you're lucky, some will be hot, and some will be cold. The leg between the last "hot" and the first "cold" in the line is the leg in which the leak occurs. If there are no hot junctions, the leak is in the first leg.
The problem is determining the order in which all the junctions occur on the line, but this can usually be accomplished using a little common sense and some more touchy-feely technology. Turn off the hot water at the boiler and let hot line cool down -- overnight is good. Then gather your friends and family together and have each person grab a "hot" junction at each point where the pipes come out of the slab. Then turn on the hot water at the last fixture on the line (the last point in the line, of course, is where a pipe comes out but no pipe goes back down). Then turn on the hot water at the boiler. Have each person yell out when his pipe gets hot, and mark the order in which they yell on your diagram (you DO have a diagram, right?).
Now you know the layout of the line, and the leg in which the leak occurs. It's an easy plumbing job to replace that leg with new pipe, but routing the new pipe may be anything from trivial to a real bear of a job. But at least you've got a real handle on what you need to do.
Of course, that will fix only one leak, and as Bob said waaay back there,
"If you have one leak now you will have more later and you will never be done with the leaks."
Keep in mind, of course, that most real estate law requires that you disclose any condition that might materially affect the value of the house. If you fix just one leak, and sell the house, you are probably obligated to disclose your fix at the time of sale. Whether you're obligated to tell the buyer that a bunch of experts have cautioned that more leaks will almost certainly follow I can't say. It's at least a "spirit of the law" thing, IMHO.
Keep us posted.
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