Certainly there are...
There are many subtle techniques to use for things like this.
Seismic isn't my thing, but I'd bet it would be ideal at detecting liquid/solid matter contrast. Shear wave extinction.
EM methods from high frequency active RADAR to low frequency passive (what on Earth we call magneto-telluric) methods could detect conductive contrast like a salty water layer. I'm assuming we're talking around Jupiter or Saturn. Uranus, Neptune, Pluto and the KBOs would probably be too EM quiet for passive methods.
Getting this fun stuff to work there is another matter.