Buy the probe moisture test kit. The kit's cost ranges from $50 to $400. You could also check with a home improvement store if you want to rent one. Look for the units with insulated probes, except for the tip, for accurate readings.
Select the area to test for moisture. The probe method requires you to perform the testing on the walls from the outside of the house. Choose the test area based on a visual evaluation such as cracks, discoloration or leakage from the siding.
Drill two small holes that are 3/16 inch in diameter and about 1 inch apart using a drill through the siding only. Do not drill past the siding. The idea behind drilling the holes is to reach the sheathing behind the siding. The sheathing is a layer of wood applied to the studs, joists and the rafters to strengthen the structure. It is considered an exterior wall.
Turn the probe kit "On." Follow the manufacturer's directions on the batteries to use and the meter calibration. Insert the probes into the sheathing through the drilled holes. Push the probes a little harder to avoid any false reading due to moisture outside or around the siding.
Note the reading from the meter. A normal moisture level is between 7 and 13. Any number above 13 requires attention.
Fill the holes with a sealant after the test. You can use neutral sure silicone sealant or acrylic latex sealant that matches the color of the siding. Never leave the holes unfilled, as they could be the entry point for the moisture.