This paper introduces a novel Energy Absorption Interferometry (EAI) technique to measure the partially coherent, few-moded response of a THz detector. Two monochromatic THz beams are generated , and the relative phase difference is changed with a fiber stretcher. The beams are individually scanned, and the complex amplitudes of the interference fringes are measured. The results of the measurements is a matrix representing the discretized cross-spectral density for each natural mode through which the system absorbs energy. The tunability of the photomixers in the apparatus allows the optical response to be measured at any wavelength, and this ability is important for grating-based systems where power coupling may vary considerably with wavelength.