The bandgap energy of InGaN alloy materials can be controlled by changing their alloy composition. Since the compositional fluctuation in the InGaN quantum wells (QWs) greatly affects the characteristics of optical devices, it is very important to understand the electronic structures of such fluctuated InGaN-QW systems for the improvement of their device characteristics. Mobility edge (ME), which is the boundary energy between the localized and delocalized states, can be an index to evaluate the potential fluctuation of carriers. Although several experiments have been proposed to evaluate the ME, they give sometimes different results. In this study, we experimentally and theoretically have studied the evaluation method of ME, and have revealed that the commonly-used evaluation method does not necessarily give accurate results. It is suggested that the method using photoluminescence excitation measurement is the best way to estimate the ME in InGaN-QWs