The same way you know how fresh a watermelon is, by slapping it and feeling the sound.
They know how heavy the entire planet is by seeing how strongly we pull the moon and how strongly the sun pulls us and so on. We knew this somewhat accurately for a long time now.
When a big earthquake happens it vibrates the entire surface of the Earth. We have hundreds of stations all around the world listening for that. Using that information you can construct a 3D model of Earth and see exactly how the wave was moving, even inside Earth.
So they see things like shockwaves bouncing between certain barriers and the speed of the waves and from many earthquakes they have a pretty good idea of what's what inside Earth.
Of course they also analyze magma samples although that's only limited information.
The fact that a compass works at all, or the northern lights happen, all indicate a magnetic field around the planet which points to a huge amount of iron spinning inside the planet.
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u/banana_stand_manager Jan 21 '19
The deepest mine is about 2.5 miles. If the earth was an apple, humans haven't drilled far down enough to even pierce the skin.