We present an electro-optic sensing system for power and electric field measurements at the W-band. An electro-optic probe was fabricated to be small enough to be inserted into a WR-10 waveguide to realize the endoscopic detection of fully guided electric fields. The intensity of the measured electric fields when propagating inside the field-calculable waveguide with respect to the applied power was calibrated. Thus, it was found that the probe can be utilized in a W-band power sensing system. The performance of the probe, based on a 94 GHz photonic heterodyne scheme, is presented.