re: They do when they use radar.
If all the info you had of Antarctica was a reasonable-res topographical map, would you truly not be able to make some pretty good guesses as to whether the surface was water or stone? (admitting that I could not)
On mars, if everywhere you look you see basalt flats filling all the depressions, is it safe to assume they are all basalt? What about if there is high correlation with the neutron mapping from Odyssey, and the depth estimates match?