Cyclobutane thymine dimer, one of the major lesions in DNA formed by exposure to UV sunlight, is repaired in a photoreactivation process, which is essential to maintain life. The molecular mechanism of the central step, i.e., intradimer C-C bond splitting, still remains an open question. In a simulation study, we demonstrate how the time evolution of characteristic marker bands (C-O and C-C/C-C stretch vibrations) of cyclobutane thymine dimer and thymine dinucleotide radical anion, thymidylyl(3'-> 5')thymidine, can be directly probed with femtosecond stimulated Raman spectroscopy (FSRS). We construct a DFT(M05-2X) potential energy surface with two minor barriers for the intradimer C-5-C-5' splitting and a main barrier for the C-6-C-6' splitting, and identify the appearance of two C-5-C-6 stretch vibrations due to the C-6-C-6' splitting as a spectroscopic signature of the underlying bond splitting mechanism. The sequential mechanism shows only absorptive features in the simulated FSRS signals, whereas the fast concerted mechanism shows characteristic dispersive line shapes.