Weigh the thistle seed in its storage container. Record the weight. This is the thistle seed's weight at 100 percent moisture.
Air dry milk thistle seed that you intend to plant in the future. Spread the milk thistle seed out on a sheet of newspaper in a warm, sunny spot for 24 hours. Place the thistle seed back in its storage container, re-weigh the seed and record the weight.
Oven dry thistle seed that you intend to consume. Spread the milk thistle seed out on a cookie sheet. Pre-heat the oven to 350 degrees Fahrenheit. Bake the milk thistle seed for one hour. Place the thistle seed back in its storage container, re-weigh the seed and record the weight.
Subtract the new weight after air drying for 24 hours or oven drying for one hour from the original weight. This is the weight lost. Divide the weight lost by the original weight. Multiply that number by 100. This is the percent moisture of the thistle seed.
Dry the seed in the oven or in the sun and re-weigh it until the thistle seed is at roughly 12 percent moisture.