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Heat management is a key concern in the development of vertical-external-cavity surface-emitting lasers. Especially, high power systems are mainly limited by their heat transfer capabilities. A commonly used quantitative measure for the heat flow in such systems is the thermal resistance. So far, the thermal resistance is usually determined by evaluating the shift rates of the emission spectrum induced by varying the heat sink temperature and the input power. Yet, in multimode operation, the shift rates at the lower and the upper wavelength limit of the emission spectrum differ. In this paper, we will investigate the connection between the emission wavelength and the temperature profile inside the gain medium. We will show that the thermal resistance corresponding to the maximum pump spot temperature can only be obtained, by considering the shift rates at the long wavelength limit of the emission spectrum. Furthermore, we will show that the roll-over temperature is independent of the heat sink temperature. Based on this finding we present a novel technique, which enables the determination of the thermal resistance without the need for spectrally resolved measurements. The new technique surpasses the wavelength shift-based method both in terms of accuracy and measurement speed.