Humidifiers should keep the humidity in the case constant. They consist of a water reservoir, usually a sponge, which is in a perforated shell or a container and needs to be moistened regularly. Some can be placed in the instrument; others have to find space in the case.
To see how wet it is in the case, some humidifiers are provided with a chemically impregnated indicator strip that changes color depending on the humidity.
The other variant are mechanical or electronic hygrometers. The latter in particular work very precisely and are integrated in some tuners together with a metronome.
On the plane
If you use a humidifier, remember to refill it regularly. This should be done especially before flights because the humidity in the airplane is very low.