Technically, theistic evolution isn't a theory. It's a religious belief. It's a belief that's widely held because many people feel that accepting the scientific theory of evolution does not conflict with their theistic beliefs.

The answer is Paleontology.

Theistic evolution is a concept that asserts that the classic religious teachings about God are compatible with the modern scientific understanding about biological evolution. In short, theistic evolutionists believe that there is a God, that God is the creator of the material universe and all life within, and that biological evolution is simply a natural process within that creation. Evolution, according to this view, is simply a tool that God employed to develop the various life forms seen today. Theistic evolutionists may also believe that humans evolved, or that they were specially created by God, and that all other life was allowed to evolve.