Take a sample. weight it. Microwave for 1 minute. Weigh it. Microwave for 30 seconds. Weigh it. Microwave for 10 seconds. Weigh it. If you get 2 weights the same the sample is dry. if not keep reheating for 10 second intervals (can do longer but might start the sample on fire).
OR you can weigh and then oven dry at 300F for 24 hours and weigh. put back in for an hour and reweigh.
The weight of the dry sample divided by the weight of the wet sample is the % dry matter.
eg: 85grams dry compared to a 100 gram wet sample = 85% Dry matter.