How did Raistlin know that he could kill Caramon if he wanted to?

Raistlin knew that he could kill Caramon if he wanted to because he had already killed what he had thought was Caramon in the Towers of High Sorcery. The final test that he had undertaken, and failed, was the test to show him the depths of his own jealousy and hatred. They showed Caramon what his brother really was so that he could understand him, and still Caramon loved his frail twin unconditionally.