Take a freshly cut piece of hard wood. It should be free of debris. Weigh it on a scale. Record the weight. This is known as the "original weight" or "cut weight."
Re-weigh the wood. After letting the wood age, weigh the wood again and record the new weight. The percent moisture is given by the equation:
(original weight - new weight) x 100 / original weight
The new weight will be less than the cut or original weight.
Continue to periodically re-weigh the wood. If you are seasoning the wood, check the weight about every week or two. When the weight does not substantially change for two or three measurements, it should be "seasoned" or in equilibrium with the prevailing environmental humidity. Remember humidity often changes with seasons depending on where you live.
Hard wood should be seasoned for exposure to the prevailing "equilibrium moisture content" for a particular region. See http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch12.pdf to find the equilibrium moisture content. The target moisture content for most hardwoods is between 10 and 15%.
Weigh a fresh-cut, one-inch long section of wood, cut along the grain. For extrapolating your wood moisture calculations to similar lumber, make the piece of wood a specific volume, such as one-inch by one-inch by one-inch (it is more accurate to use a larger volume of wood then divide by the total cubic inches in the steps below).
Place the wood in an oven at about 220 degrees F. Wait about eight hours.
Re-weigh the wood and record the weight about every four hours, then replace in the oven. When the wood no longer has an appreciable weight loss, it is completely dried out. Record the dry weight.
Calculate the correct equilibrium moisture content weight per square inch. Average hardwoods are about 1.5 more dense than water, and lose about 50% of their weight between original weight and dry weight. To target equilibrium moisture content, the wood should weigh about 10 to 20% more than the dry weight. Example: if you dry weight is one ounce per square inch, the wood should weigh 1.1 to 1.2 ounces for equilibrium moisture content weight.
You can now apply this to board lumber to determine moisture content. Weigh the lumber, figure the actual volume of the board in square inches (not necessarily board feet). Divide total weight of the lumber by the number of square inches. This is the density of the wood. Calculate the equilibrium moisture content of the wood per square inch from Step 4. If the lumber is more dense, it needs to season longer.