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Off-site fabrication allows for efficient production and construction, while the prestressing process enhances the load-bearing capacity of structural components. Due to these advantages, the application of prestressed prefabricated structures increases significantly. However, various influences present at the early stage of fabrication, such as pouring conditions, friction with formworks, and early-age cracks, may cause differences between designed and real values of prestress forces, thereby affecting the bearing capacity and durability of prefabricated components. These differences are often reflected in the strain field. Therefore, it is of interest to monitor the performance of prefabricated structural components at early stages, that is., before, during, and after prestressing, by studying the internal strain distribution. This article aims at developing a methodology to identify prestress losses under early-age cracks in prefabricated prestressed beam-like concrete structures with a complex geometric cross-section and validating the application on a double-T slab of a five-floor garage at Princeton University. Embedded long-gauge strain sensors are used to monitor the strain at different locations. The focus of this article is on the analysis of the sensors embedded in the slab’s longitudinal direction (longitudinal sensors). The main challenges of this research include the non-linear strain distribution in the complex cross-section of the structures, which makes the Bernoulli hypothesis only partially valid, the uncertainties of geometric and mechanical parameters, and the effects of early-age crack opening on the evaluation of prestress forces. The developed methodology, based on the measurements of strain distribution before, during, and after prestressing, enabled the identification, that is, detection, localization, and quantification of prestress losses under early-age cracks in the prefabricated slab. The findings of this study have important implications for the design, construction, and maintenance of prefabricated structural components, enabling enhanced safety and durability throughout their service life.