A damp or moisture meter is a simple tool is normally equipped with two steel pins that can be used to detect the amount of moisture in floorboards. Meters can be purchased from most large tool/diy shops and are easy found on Amazon starting from around £20. The basic type checks for moisture by measuring the electrical resistance between two pointed metal probes which are pushed into the floorboard. There are some more expensive meters on the market that use radio frequency emissions but these are probably not needed by the diy homeowner.
Checking moisture in floorboards
It is always a good idea to check the moisture content of old floorboards as anything around 20% can be a problem especially if the flooring is not getting airflow such as when air (ventilation) bricks are blocked or filled in as some people mistakenly do to stop droughts coming up through the gaps in old floorboards. Also floorboards fixed on top of concrete floors with no moisture barrier or battening can cause dampness due to condensation or raising damp.
One last note, If you are wondering what happens to your lovely old floorboards and joists when subjected to damp and no airflow just think about how quickly your old gym kit starts to go mouldy and smelly if you forget to take it out of your gym bag.