While optical detection of water adsorption is certainly the
most common, may I mention acoustic detection ?

If you are versed in RF electronics (and actually not much is
needed nowadays with monolithic amplifiers provided by Minicircuits
and Macor), you can go for a cheap Rayleigh mode Surface Acoustic
Wave filter. These are used as RF bandpass filters: the propagation
of the Rayleigh acoustic wave is strongly attenuated by the
adsorption of water. By gluing a small thermocouple on top of the
SAW device, you just have to cycle the temperature of a Peltier
module on which the SAW device is fixed, and log the temperature for
which the acoustic wave propagation disappears.

I build such a system for fun during my PhD, although I didn't do
any calibration on it (others have). As mentioned, the contamination
of the surface is a major issue for more than in-lab use.

Jean-Michel