Charles, it is definitely true that you need "to maintain the guitars with humidity". I'm not qualified to talk on this subject with authority but I have seen enough experts in the field stress this that I believe it to be true. As an example, I once saw Gregg Voros, Master guitar tech at Gruhn guitars and Instructor on Learn and Master Guitar Set-up and Maintenance, demonstrate guitars damaged by it in a video.

I don't know that you need to, or if there is a method, measure humidity in a guitar. Monitoring the humidity in the environment, in the room your guitars are in, should be adequate. I live in a part of the country that is brutally cold and dry in the winter and miserably hot and humid in the summer. I just bought a cheap indoor thermometer/hygrometer of Amazon. I think it was less than $20 and seems sufficient for my needs.

I didn't go to the methods you linked. I use a room humidifier.

Which reminds me!!! It's time to get the dehumidifier out! Where did I put it last fall?!?!