No, there is not a visual aid; you must measure or calculate the moisture.
Measuring with a meter
To measure the moisture directly, you can use a moisture meter. Pin-type meters require you to drive a set of pins into the wood, while pinless meters just need to be pressed against the surface of the wood.
Delmhorst, a manufacturer of both types of meters, explains the pros and cons of each type on their website:
For wood, both pinless and pin-type moisture meters have their uses.
Pin-type meters, in particular, have the advantage of being able to
tell you the depth at which a moisture pocket in wood occurs. To do
this, simply push the pins in partially, take a reading, push them in
a little more and take another reading, repeating until you hit the
moisture pocket in the wood or reach the maximum penetration depth of
The major drawback to using pin-type meters is that they can only
measure the moisture content of the material between the pins, so if a
moisture pocket falls outside of that area, it can go undetected.
However, this same focus is what allows pin meters to give users a
precise location for moisture pockets and the depth at which they
Pinless meters, on the other hand, are very good at scanning large
areas of an object quickly. With these meters, there are no pins to
constantly and carefully push in and out of the wood. Simply press the
scanning plate against a flat piece of wood, push the button, and you
have a moisture reading in seconds. This process can be repeated over
different areas of a single piece of wood to give a thorough reading
of the %MC of that piece of wood without leaving pin-holes in it.
There are, however, a few drawbacks to pinless meters:
A pinless meter requires a flat surface on which to press the scanning
plate. Without such a flat surface, readings cannot be taken.
Pinless meters have a fixed scanning depth. If a piece of wood is too
thick, the pinless meter will not be able to take a reading of the
deepest layer of the wood. If the wood is too thin, you may actually
get readings of the material behind that piece of wood, rather than
for the wood itself.
Metal objects in piece of wood can cause a false positive. Pinless
meters emit electromagnetic waves to scan a piece of wood, and use the
distortions of returned waves to calculate the %MC of that wood.
Foreign objects such as nails interfere with the electromagnetic waves
emitted by the meter, creating a false positive reading.
When choosing a type of moisture meter for woodworking, take into
account the size and shape of the wood you’re working with. For
example, floorboards and other thin pieces of wood might not be thick
enough to get reliable readings from a pinless meter, whereas big
lengths of lumber would be perfect for pinless moisture meters.
Calculating moisture content
To calculate the moisture, you take a sample of the wood and weigh it. Record this as the initial weight. Then dry the sample in an oven set to 217 degrees F for 4 hours and weigh it again. Repeat this step until you've recorded the same weight several times in a row. This final number is the oven dry weight. Then use the following formula:
moisture content = (initial weight - oven dry weight) / oven dry weight * 100
More specifically, a study from Oregon State University recommends the following procedure:
Performing the Test
It is simple to do an oven-dry test, but the devil is in the details. The test is
performed by cutting the sample to be tested, weighing it, drying it to a constant weight,
reweighing, and doing the calculation. Now the details.
The samples need to be representative of the lot of wood from which they are
taken. Thus, they should come from throughout a pile or shipment, not just the top or
sides. After a board is chosen, the sample should be cut at least 2 feet from the end of
the board because wood picks up and loses moisture very rapidly through the end grain.
The samples should either be weighed immediately after cutting or each sample
should be stored in a separate plastic bag. After the initial weight is obtained, no
particular precautions for storage need to be observed and the sample can be oven-dried
at your convenience.
Either a data sheet with sample numbers should be used or sometimes the
weight can be written directly on the sample. Once the initial weight is taken and the
sample begins to dry, there is no going back so it's important to keep a good record.
The time to dry is typically about 24 hours for a one- to two-inch sample cut from
dimension lumber or boards. The correct method is to dry to a constant weight, meaning
that the weight change over a four-hour period is less than twice the sensitivity of the
scale. This means less than about 0.2 grams for a 100 gram sample.
Do not add wet samples to the oven when other samples are almost dry. Water
will evaporate from the wet samples and be picked up by the drier ones causing an error
when they are weighed due to a temporary increase in moisture content.
If the oven temperature is too low, the relative humidity in the oven will not be
low enough. The ambient air contains moisture and raising its temperature simply lowers
its relative humidity causing a lower equilibrium moisture content. For example, if the
ambient air is at 70°F and 50% relative humidity, the relative humidity in the oven will
be about 1% with an equilibrium moisture content of about 0.1 to 0.2%. On a humid day
the samples won't dry as much, but at 217°F the variability due to ambient conditions
is minimal. An oven in a humid tropical country will give results that are within 0.3 or
0.4% moisture content of and oven in a cold dry climate as long as 217°F is used.
If the oven temperature is too high, some of the wood components can be driven
off. When these are counted as moisture, the indicated moisture content is too high.
Do not overload an oven. Good air circulation around the samples is needed to
reduce the risk of fire.
Is it dry enough?
Whether you measure the moisture of your lumber with a meter or by calculating it yourself, you might find general moisture content recommendations ranging from 6%-12%. The truth is, this figure will fluctuate with temperature and relative humidity. What you really need to aim for is the equilibrium moisture content, or EMC, which is the moisture content at which the wood is neither losing nor gaining moisture from the air. Suffice it to say, this is not quite as simple a formula, but there are several calculators and charts you can use, such as the following:
If your wood has not reached equilibrium, you need to let it continue to dry. Ideally this involves stickering the lumber and air-drying it or drying it in a commercial kiln or a homemade solar kiln. The typical rule-of-thumb for air-drying fresh-cut lumber is 1 year per inch of thickness, but you can continue to check the moisture content periodically since the type of wood and environmental factors can accelerate or lengthen the drying process.