A simple way with reasonable accuracy
The water content of honey is now considered to be more important than it once was. In my early years of beekeeping it was common practice to turn a comb of unsealed honey on its side and give it a hefty shake. If the nectar flew out it wasn't ripe, so it was usually extracted before the rest was uncapped. If no liquid came out of the comb it was low enough water content to be extracted.
Now the allowed water content is stated in regulations, beekeepers are encouraged to measure it if they sell their honey. This is usually done using a relatively cheap instrument called a refractometer, which many beekeepers now own.
The refractometers that are usually bought by beekeepers (who often go for the cheapest!) aren't particularly accurate, but are within about 1% or so. The scales are often a bit "fuzzy", but in my experience this is because far too much honey is often put on the slide. It only needs a small amount and the line will be clearer.
The calibration of refractometers needs to be done regularly, as on most types there is an adjustment screw that can be easily turned in use, so changing the reading without being noticed. Calibration oil is available, but beekeepers have found that some oils that are available in the kitchen are remarkably stable.
On August 31st August 2010, I emailed our Wisborough Green Chairman, Tom Moore, who I knew had just purchased a new refractometer that he had calibrated with oil that was supplied with the instrument. I said I was told that olive oil is reckoned to be 26.5% water and sunflower oil is 25.2%. I asked Tom to do a check.
Tom's response was:-
"We just checked as follows, using what we have in the kitchen on our refractometer, which we re-calibrated with its own oil a few days ago.
Sunflower oil (Sainsbury's) 25.0%
Olive oil regular (Sainsbury's) 27.2%
Olive oil regular (Bertolli) 27.2%
Olive oil, Spanish extra virgin (Sainsbury's) 27.0%
Olive oil, Italian extra virgin (Filippo Berio) 27.0%
I would have thought there could be considerable variation in batches of oil, but these look consistent, if not exactly on the figures you give. It also has to assume our refractometer is accurate. They are all read on the honey scale, which only goes as far as 27 so we guess the points above."
On 31st January 2012 Tom sent me another email, saying he had just re-calibrated his refractometer and his current bottle of Sainsbury's sunflower oil read 25.2%
It seems from the above that by using oils that are in most kitchens the refractometers that are available to most beekeepers can be "calibrated" to within around 0.5%.
On Thorne's website in 2016 it states "An aid to calibration. Using medicinal liquid paraffin smear a small amount on the prism. This should calibrate to 24.5% on the water scale. Once the refractometer is set/adjusted to this figure all honey samples will also show the correct water content."
On 1st September 2015 I received the following email from Brian P. Dennis of Northamptonshire.
Roger, Following is my note on calibrating a refractometer.
"Calibrating a refractometer. Owing to the remarkably consistent properties of Extra-Virgin Olive Oil, one drop of it on the slide will always read between 71 and 72 on the Brix scale. If you set the lock-nut to show any such oil at 71.5, you will have correctly calibrated the water content scale at the same time. Regards, Brian."
So, there is another way of doing it.
Refractometers used by beekeepers are for guidance only. The figures mentioned above are for reference and are not intended to replace the conventional methods of calibration.