Premature rain erosion damage development at the leading edges of wind turbine rotor blades impair the efficiency of the turbines and should be detected as early as possible. To investigate the causes of premature erosion damage and the erosion evolution, test specimens similar to the leading edge of a rotor blade were modified with different initial defects, such as voids in the coating system, and impacted with waterdrops in a rain erosion test facility. Using CT and XRM with AI-based evaluation as non-destructive measurement methods showed that premature erosion arises from the initial material defects because they represent a weak point in the material composite. In addition, thermographic investigations were carried out. As it shows results similar to the two lab-based methods, active thermography has a promising potential for future in-situ monitoring of rotor blade leading edges.