The most effective way to measure moisture within a substrate is to use a moisture meter. This is usually carried out by professionals. Moisture meters have LED displays which show the moisture content, usually in % MC. ASTM, the American regulating body, details 5 test methods. The plastic sheet method involves taping a clear plastic sheet to the surface and examining it for moisture after at least 16 hours. Other methods include using specialized in situ probes and infrared thermography.