How to I check for slab leaks?

Hi all! This is my first post here. My husband and I are about to install a new tile floor in our kitchen that we are remodeling. When we were doing the drywall and had the cement slab covered with plastic, we noticed that a couple of areas were damp when we took the plastic up. Not a lot of water or anything, just a slightly discolored area where the cement was damp.
Is this normal to have a small amount of moisture? Our concern is that when we installed tile in our bathroom, the grout discolored in one area and we think it may be from moisture there. We don't want to have this happen in the kitchen. And we are also worried that it might be an indication of problems to come.
So is there any way to check for leaks under a slab?
Our house is about 40 years old. We haven't any idea what if anything needs to be done about this. But would rather do it now than when the remodel is complete.
Also, is there anyway to put some kind of moisture barrier down on top of the cement that we can tile on top of? And would that be neccesary?
Any help from all you pros would be appreciated.
Kim
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Plastic is an great method of attracting and condensing moisture. Cover the area with paper or cloth and wait a couple of days. If the paper/cloth gets wet then you have an opportunity to find the leak.
You want to bond to the concrete not be separated from it.
The only way I know of to locate small leaks under the slab is either with an roto hammer and start making holes or make sure all of the faucets are off and use an stethoscope to listen for running water. All of my slab leaks were major and took no time at all to figure out that there was a problem.
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