The fatigue safety of the reinforced concrete (RC) deck slab of a sixty-year-old steel-concrete road viaduct is questioned based on the results of code-based “re-calculation.” Since strengthening intervention would lead to major costs, detailed investigation of the fatigue safety was performed using data obtained from “pocket monitoring” which is a rational and ready-to-use monitoring device for bridge engineers. The implemented “pocket-monitoring” concept for the determination of fatigue action effects in the RC deck slab is outlined. Results in terms of fatigue relevant stresses in steel rebars of the RC deck slab reveal that the fatigue safety requirements are largely fulfilled based on the proposed verification method using directly data from monitoring. In addition, uncertainties related to the “pocket-monitoring” concept, and in particular the influence of temperature effects, are outlined and analyzed.