How Do We Know What the Earth's Core Is Made Of?

Humans have often wondered about what lies at the center of our planet and have considered hidden civilizations, hell, dinosaurs and interior oceans among the possible candidates.
Within the past 100 years, great strides have been made in the study of the earth's core, and much to the disappointment of sci-fi fans, it appears that it is nothing more than an outer shell of liquid iron surrounding an interior of solid iron (mixed with sundry other elements).
Yet since no one has ever been to the center of the earth--due to, among other things, raging temperatures and an impassable surface mantle--how do we know it is iron?

The answer is waves. Though seismic activity in the form of earthquakes is often very destructive to life on the surface, with far-reaching consequences for humans, they allow a select group of scientists to study the center of the earth.
The few who study the center of the earth belong to SEDI: The Study of the Earth's Deep Interior. They analyze the center of the planet based on comparison with what we know about other planets in the solar system and the result of seismic waves as they travel through the core and out the other side.
Seismic waves can tell scientists a lot about what materials make up an object as they pass through them and help identify most metal elements based on the rate the wave moves through them. While there were strong suspicions in the scientific community starting at the end of the 1800s that the earth's core was made of iron based on our magnetic field and the rotation of the sun and the moon, it wasn't confirmed until 2002 when the tests were refined.

However, there is still some debate on the liquid iron composite that surrounds the core, and questions as to why it appears metal crystals on one side drastically differ from crystals on the other.
In fact, while seismic wave tests are clearly the best option geologists have for answering the question of what the core is like, it is an imperfect science that requires all factors to be aligned perfectly to get reliable and acceptable results. Due to the unpredictability of seismic activity and worldwide cooperation that needs to be in place to get the best results, this method is shaky and often called into question.
However, until scientists are able to create waves of their own that match the power and ability to penetrate as deep as seismic waves, we have no better means to determine what the center of the earth is made of.