ceres - Subterranean Oceans On Other Planets/Planetoids: How Do Astronomers Deduce This

How they have concluded that there could b.e a subterranean ocean on Ceres is by Spectralscopy.

Spectral Signature can be summarised to like this:

Different elements emit different emission spectra when they are
excited because each type of element has a unique energy shell or
energy level system. Each element has a different set of emission
colors because they have different energy level spacings. We will see
the emission spectra or pattern of wavelengths (atomic spectra)
emitted by six different elements in this lab. We will then identify
an unknown element by comparing the color of the unknown with the
flame color of our knowns.

And another:

When something is hot enough to glow (like a star), it gives you information about what it is made of, because different substances give off a different spectrum of light when they vaporize. Each substance produces a unique spectrum, almost like a fingerprint.

So how the scientists would have concluded that there was water because of the water vapours in the atmosphere. It gives off a certain wavelength that could be compared the closest element that gives off the same wavelength which is water.

Then the scientists would have used Galileo's magnetometer(an instrument which measures the strength and direction of magnetic fields) to conclude that there could be an ocean on the dwarf planet. The strength and response of the induced field would tell the scientists a rough estimate of the dwarf planet's sub surface which in return they could deduce the fact the there is a large amount of water present which equals an ocean.