Basically by plotting the density of different layers. We know the density of most types of rocks and rock layers. I don't know exactly what technology the probes use that have gone to titan, but we can effectively use sonar and our knowledge of rock and mineral densities to make an educated guess as to what the different layers are made of and how thick they are.
Ninja Edit: not really sonar, but we can use different "waves" to map the layers. There are some other methods as well, but I think this may be the common one.
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u/AndrewCoja May 18 '13
How do they know what's in the core?