We present recent experimental verification of a new in-situ fiber optic sensors for detecting the curing of thermoset composites. This device is sensitive to the difference between the refractive index of the curing polymer matrix within the composite and that of a fully-cured reference fiber made of the same thermoset material. The method is indifferent to temperature variations imposed during the cure process. The process of polymerization usually results in an increase in the refractive index of a thermoset resin. Increases on the order of one percent are common. In our method, a fiber optic core, comprising the fully-cured neat resin, is placed within, and in direct contact with, the same curing thermoset composite matrix, which functions as a cladding. When the matrix is not fully cured, its lower index of refraction permits waveguiding in the fully-cured core. As the composite is subjected to curing conditions, the matrix cures and the index of refraction difference diminishes. At final cure, the waveguiding characteristics of the fiber optic cure sensor are lost, resulting in a vanishingly small transmitted intensity, at any cure temperature, and without calibration. Our sensor does not suffer from the limitations exhibited by other methods, in large part because it is self-referencing, and relies on a differential rather than an absolute measurement. Our technique is sensitive to the extent of cure even at the latest stage and could be used within the body of large composite structures undergoing a curing process in an autoclave. It is further expected that because of its small size, the sensor may be left in the final product without detrimental effect to the eventual application.