We show that the thermal measurement with a vertical temperature gradient is an important tool for detecting magnetization structure with high sensitivity. In the case of the longitudinal spin Seebeck effect (LSSE), Pt/YIG is one of the most important bi-layer structures for studying pure spin current phenomena. The spin current can be converted to the charge current by the inverse spin Hall effect (ISHE) with a defined spin direction given by the magnetization orientation of YIG. However, both the ISHE voltage from the LSSE and the magnetoresistance from the electrical transport show a clear plateau behavior in the low field range, which is inconsistent with the magnetization reversal behavior of YIG. By using thermal spin current and the highly sensitive micro-magneto-optic Kerr effect (MOKE) measurement, we provide conclusive evidence that the hitherto unaccounted for peculiar field dependence is due to noncollinear magnetization between surface and bulk YIG. Our results show that LSSE with a vertical temperature gradient can function as sensitive tools to study the surface magnetization configuration of materials.