How to measure a dough humidity?
You don't need anything special, just a scale. Put a trivet, then a pan on a scale and tare it. Next, pour in a certain amount of flour, say 100 grams. After that heat the pan and therefore the flour up on the stove, which will evaporate the water, then re-weigh (that's why you have a trivet: you don't melt your scale). The difference between measurements will tell you the humidity of your flour. 

I personally don't think there's much to be gained knowing this. Recipes are written accounting for water content in flour, they don't expect flour that's completely dry. I've baked a lot and I've never had a situation where the humidity of the flour has made a noticeable difference, especially when considering other environmental variations.