In the last decade, polymeric-based composites, in light of their low weight/mechanical strength ratio, have been widely used in many industrial areas, such as automotive, aeronautic, and aerospace areas. Because of the dependence of their properties on the manufacturing stage, real-time monitoring of the processing stage has been indicated as the key point for improving the quality and reducing manufacturing process costs through an intelligent control of the manufacture itself. To this aim, optimal monitoring systems should be non-intrusive, real-time, and able to measure a physical property changing during the process development. Refractive index is a suitable state parameter being directly correlated to the material density. In this work, the assessment of the performances of a fiber-optic refractometer has been presented. Experimental results demonstrate the capability of the sensor system to monitor the cure kinetics of a polymeric thermoset and to measure its glassy transition temperature.