I have young children in my home and we live in Kansas where it can get bone dry in the winter. I have a whole house steam humidifier by Honeywell, but it can barely keep the house at 30%. And it's kind of inconsistent depending on the number of vents in the room, etc. So when it's really dry, I keep guitars in the case and a hygrometer as well. Just a cheap digital plastic one from Wal-mart for less than $10, but I understand the quality and accuracy is quite inconsistent in these cheap versions. Nonetheless, I put a lightly damp sponge in a perforated ziplock bag in the case and monitor it. Needs to be redampened every week or so. Its tough to control the humidity in the case as well - it can get too humid. I've see it go as high as 50%.
For the guitars that I like to play regularly and not mess with cases, I have a separate room that has its own humidifier on top of the whole house humidifier. I'm able to control humidity in that room much better, but it requires me filling the water almost daily.
None of my guitars are taken out of the home and everything is pretty much climate controlled, but I've bought everything used so I do not know the care from the previous owner(s) unfortunately.