The central safety factor (q(0)) during sawtooth oscillation has been measured with a great accuracy with the motional Stark effect (MSE) system on KSTAR and the measured value was similar to 1.0 +/- 0.03. However, this measurement alone cannot validate the disputed full and partial reconnection models definitively due to non-trivial off-set error (similar to 0.05). Supplemental experiment of the excited m = 2, m = 3 modes that are extremely sensitive to the background go and core magnetic shear definitively validates the 'full reconnection model'. The radial position of the excited modes right after the crash and time evolution into the 1/1 kink mode before the crash in a sawtoothing plasma suggests that q(0) >= 1.0 in the MHD quiescent period after the crash and q(0) < 1.0 before the crash. Additional measurement of the long lived m = 3, in = 5 modes in a non-sawtoothing discharge (presumably q(0) >= 1.0) further validates the 'full reconnection model'