1. The average density of earth is more than would be possible if there was only crustal material so there must be something else.2. Following from 1, the gravitational force produced by the earth is too large.3. Seismic waves can travel through the internal area of the earth at faster speeds that they would be able to if there was no liquid/solid internal material.4. The pressure inside the earth due to gravity is too strong for there to be a hollow area, if there was gas it would be compressed to a solid anyway. 5. The magnetic field; there is nothing in the crust which produces a strong enough magnetic force to explain the magnetic field. The only possibility therefore is that it is generated by a core.6. Plate tectonics; there needs to be some form of driving force for them and something to support them from below, there also needs to be an internal source of heat to drive the process. In this section you can also include volcanoes and earthquakes as they are both primarily driven by tectonic processes.That's all I can think of off the top of my head right now, I'm sure there are more.