For the experimental investigation of existing concrete structures it is crucial to be able to analyze and assess the load bearing behavior online during the loading test. Only this ensures that a pronouced failure or an inacceptable damage of the structure can be avoided in the test. Unlike bending failure, this assessment is rather difficult and not yet securely possible for low-ductile types of failure, like the shear failure of reinforced concrete structures members without web reinforcement. This paper explains the difficulties of assessing the danger of shear failures of reinforced concrete structures und discusses which main influencing factors have to be considered for an evaluation and especially for the case of a loading test. For the experimental tests, which have been carried out, different measuring techniques (usual deformation measurement techniques, photogrammetry and acoustic emission analysis) were combined and tested regarding their suitability for the detection of smallest structural changes.