From G. Pease
At 10% water (by weight), a tobacco is going to seem very dry. If the moisture level is increased to 20%, it will be quite damp. Ideally, moisture contents between 13% and 18% are right for most blends and most smokers. Some heavily sauced aromatic tobaccos are reputed to have non-tobacco content that approaches 40%. Not all that moisture is water. Various humectants (humidifying agents) are used by some manufacturers to preserve moisture levels at the desired percentage.
Q: How can I determine the moisture content ?
There are destructive methods to measure it accurately, but it isn't really necessary to know the precise moisture content unless you're just the curious sort. As mentioned above, at about 10% and below, tobacco will be quite dry feeling, and the strands will tend to break when handled. At about 12-13%, the strands will be pliable, and will endure more vigorous handling without damage. If you press the tobacco into a ball, and it stays compressed, it's over 18-20% - too moist for proper smoking. In the 15-18% range, the ball will be springy. Once you find your preferred moisture, you'll be able to tell by feel whether it's there or not.