In-situ load testing of reinforced concrete (RC) structures is often performed to confirm the presence of the required resistance for the intended use (Conformity Load Testing) or to support the assessments of the residual capacity by models (Supplementary Load Testing for Condition Assessment). When performing an in-situ load test, one of the main concerns is the avoidance of irreversible damage to the structure, since that may impair the structural performance after testing and eventually reduce its residual service life. Therefore performance indicator are needed to recognize the development and/or to prevent failure during an in-situ load test of real structures. For many of the existing RC structures without transverse reinforcement, shear failure is one of the governing mechanisms. Due to its brittle character, this failure mode is difficult to sense at an early stage of its development. Up to now no good performance indicator is available for shear. In the presented study, a smart key performance indicator for in-situ load tests has been developed, with particular focus on shear failure. Results of several laboratory tests on full- scale shear beams have been examined, confirming that a change in structural behaviour at an early stage during testing can be traced by the proposed indicator. With the proposed performance indicator, performing measurements only on the top or bottom of the tested specimen is necessary, which is of importance for the applications to existing structures, with limited access for measurements. An algorithm for data analysis has been developed, which can be used to control the proposed performance indicator in real time during testing.