The use of soft and flexible materials in rehabilitation support systems based on robotic exoskeletons plays a crucial role in ergonomics, reducing weight and complexity. However, the selection of these materials can have mechanical implications that affect the performance of the exoskeleton in the chosen application. This paper proposes a testing protocol to assess the mechanical and structural features of a prototype upper limb robotic exoskeleton using an optical motion capture system as the primary measurement instrument. In addition to the protocol, a literature review is presented, laying the ground for the theoretical development of the mechanical evaluation for prototypes. Also, the materials used concerning the evaluation technologies required for data collection and assessment of the devices are described. The development and future application of this protocol will enable the identification of the relevant motion parameters on the exoskeleton prototype, the changes that its structure may experience due to the use of the flexible materials, and the effectiveness of the mechanical support provided in the upper limb. Finally, the proposed protocol can be extended to other types of robotic exoskeletons built with similar materials, regardless of their final application, extending its applicability to other study cases.