There are several different ways you can try to detect mold in hidden locations, however most of them are considered invasive procedures that will cause damage to the finished surfaces.
Intracavity wall and ceiling samples are going to give you the most accurate information of all the methods, but it should be realized that it will not pinpoint the problem to an exact location.
Another method is to use a carbon dioxide detector…note I said dioxide, not monoxide. Molds use oxygen in order to grow and give off carbon dioxide as a byproduct just like people. Drilling holes in the wall and inserting the sensor will give you data where you can map out any high areas that are suspect and later cut larger inspection holes.
Mold growth also generates a small amount of heat as the colony grows. You can cool the room down and look for heat signatures depending on how sensitive your infrared camera is and the resolution.
An experienced mold inspector should be able to look at the home and find conditions conducive of mold growth. Sometimes previous repairs have been made to correct the contributing factor that the homeowner does not tell you about, or has no idea were undertaken. Look at the age of any suspect areas and try to determine if a recent repair has been made. If the materials do not appear to be original in an area, there is a good chance that changes have been made. Begin looking in the home where water might have gotten to from any suspect areas. Keep in mind that moisture can come from leaking plumbing supplies or drains, air conditioning condensate lines, improper ventilation that result in condensation formation, missing or insufficient insulation, and the way an occupant lives in the space.
People are always worried about ripping into finished walls in order to find mold growth. For some reason, everyone thinks that wall surfaces are more expensive than gold. Remind clients, drywall is cheap and easily replaced if they really want to get to the bottom of the issue once and for all.