Non-destructive testing (NDT) methods to determine the condition of existing concrete members, such as pre- or post-tensioned beams belonging to concrete bridge decks, are used in assessing the need for preventive maintenance, repair or replacement of the structure. In this work, a prestress force identification method for concrete beams using static deflections is presented. The procedure makes use of the Euler-Bernoulli beam model and it is able to estimate the prestress forces using one or a set of displacements measured along the beam span. The method also requires the knowledge of the flexural rigidity of the element under examination. The deflected shape of a post-tensioned concrete beam, subjected to a three point-bending test, was short-term monitored in several laboratory tests. The accuracy of the deflection measurements allowed to obtain good prestress force estimations. Finally, it was shown that “compression-softening” theory holds for post-tensioned concrete beams with prestress forces satisfying the decompression serviceability limit state. Results of this experimental work will be presented and discussed.