Implementing exoskeleton robots for remote motor rehabilitation offers a solution for the increasing demand for an ageing and growing society, providing effective and consistent treatment service, reducing therapist workload, and potentially being more cost-efficient, especially up against pandemic reality. This study examined a validation process of the applicability of the upper extremity’s range of motion measurement with the custom-designed, 3D-printed, passive 5-DOF exoskeleton supplied with optical rotary encoders. Four healthy participants performed 2 exercises involving complex shoulder and elbow joint motion while wearing the exoskeleton. Data obtained from the exoskeleton was compared to data from a Motion Capture System (MSC) used in the previous study. Significant dissimilarities in mean values of achieved ranges, velocities, and accelerations were observed for various joint angles; however, satisfactory results were obtained at plotted scopes levels. The study discussed possible reasons for differences in joint angle measurements between the exoskeleton and MSC methodologies. It highlighted obstacles related to the construction of the exoskeleton and differences in participant movement patterns.