If you are concerned then get a hydrometer. https://www.elderly.com/collections/humidification/products/oasis-oh-2-hygrometer
Interesting idea. How well does it keep humidity up around 50% in the winter? Is it consistant? How often do you have to change them or put more water in?
A hygrometer just measures humidity, it doesn't do anything to change humidity levels. To change the humidity you'd need a humidifier, either a small one that fits in a case and humidifies the ukulele, or a larger one that would humidify the room. I think the point of the hygrometer suggestion was to take the guesswork out of knowing what the humidity in the room (or case) actually is.