I am assuming you are asking about measuring the salinity of water remotely, which would exclude landing a probe to directly measure the salinity of water on a celestial body.
The remote measuring of surface salinity of Earth's oceans via satellites is being investigated, the process uses microwaves, with a frequency of 1.4 GHz or wavelength of 21 cm (L-band).
If this was combined with spectroscopy, to identify salt minerals, the plumes of geysers erupting from celestial bodies could be analyzed to determine the salinity of water that was the source of the geysers.
Edit 14 June 2023
Megaplume of water vapor erupting on Enceladus caught by Webb Telescope
Plumes of water vapor erupt from Enceladus via cryovolcanoes that form over cracks in the ice. These plumes can extend hundreds of miles from the surface. When a team of NASA researchers looked closely at the new JWST data ...
Webb’s observations were done using the Integral Field Unit (IFU), which can simultaneously image an object and see the spectra it is giving off, which tells us what substances make up the object. The IFU is part of Webb’s Near-Infrared Spectrograph (NIRSpec) instrument, which can see molecular emissions across a broad range of the infrared spectrum. Infrared emissions coming from the plume revealed not only that it was made up of water vapor, but how far that water vapor extended from the surface, which revealed its immense size.lockquote