The possibility to measure strains continuously using distributed fiber optic sensors (DFOS) offers enormous potential for structural health monitoring. Cracks can be automatically detected, localized and crack widths calculated. To address the relevant questions of choosing the right DFOS and appropriate application technique for monitoring existing structures, two 4 m long reinforced concrete beams were loaded under service loads in a 4-point bending test. The DFOS were either bonded directly to the smooth concrete surface or along a groove. For comparison, another DFOS was integrated into the specimen. It is demonstrated that with the used adhesive, a good strain transfer from the specimen to the DFOS can be ensured even with subsequent installation. In order to detect all cracks with high reliability, robust DFOS with a monolithic cross section should be preferred for the practical use. The accuracy of crack width measurement was verified through Reference measurement via digital image correlation. For the monolithic DFOS, all measurement deviations were within the tolerance range of ±0.05 mm. With layered sensing cables, significant misinterpretations occurred due to strongly damped strain curves.