A moisture meter that is used for pot plants may also be used in the bin, but I find feeling the bedding is just as accurate. Worms will die if they get too hot - moisture and ventilation can be used to lower the bin temperature if necessary. 6 would be the lowest level, and 7 the highest level. The presence of air within the bedding is also an important issue in correct maintenance of your worm bin. Worms are living creatures, and they need to breathe air.
Air is also important to the decomposition of any scraps placed in the bin.
Gentle stirring of the top layer of the bedding allows any accumulated gases to escape from the bottom of the bin, as well as helping in the prevention of bedding compaction. This also fluffs up the bedding material, trapping air within.