Moisture meters can be very helpful in finding concealed mold and water damage. They can also be helpful in determining if a leak is active or a past problem by measure even slight variations in moisture content of materials. There are two types of meters for materials. One is a pin type meter like the one shown. These are usually too expensive for homeowners as a good one can cost more than $400. Inspectors should have these are part of their tools. The pin type meter is especially good for testing wood materials that have been wet previously, but are being dried out. I prefer to make sure wood dried to at least 10-12% moisture content before recovering with other materials.
A surface meter is another type of moisture meter that can often be purchased for less than $50. While these may not last long or can have odd misreadings, for the investment, they can often be helpful for homeowners. Despite the use of some pretty expensive equipment to find the source of water in a leak in my house, this was the type of tool that ended up being the most helpful.
This is one that is similar to several that I have used over the years. Make sure to buy one that has settings for various materials that one might be measuring. This one has separate settings for hardwood, softwood, drywall and masonry. Always make sure you are using the right setting for the material being tested. Look for variations, as some materials will have a equilibrium moisture content that is more than 0% typically. For tile use the masonry setting.
Read reviews and look for the most accurate calibrations.
Know that every materials will have a different 'equilibrium' number. This can vary by ambient humidity in the air as many materials take in some of the ambient humidty.
Drywall should be very low...0-3%
Wood varies by species, location and weather at the time of testing. Charts are available online through the U. S. Forest Products Lab.
Tile, masonry and concrete vary widely. Look for what is typical for the material you are testing, and then look for variations that deviate from the norm. Those places where deviations are seen consistently, might be worth opening up for a closer look. Be careful to read instructions, however. Some materials such as metal screws, straps, nails or wires can affect readings creating deviations as well.