Hot-carrier solar cells offer potential for enhancing the energy-conversion efficiency of photovoltaic devices, but their design and operation require a good assessment of carrier temperatures. Electrons and holes may have different temperatures, for instance because of their effective mass mismatch in III-V compounds. We propose a purely optical method which allows the direct and distinct estimation of electron and hole temperatures in steady state. This technique, based on photoluminescence, relies on the precise determination of the band-filling signature. We apply this technique to an InGaAsP single quantum well. Electron temperature surpasses 1000 K at largest excitation intensity, while holes remain colder, close to lattice temperature. Nonetheless, the increase in hole temperature is too large to be explained purely by photon absorption, which demonstrates an energy transfer from electrons to holes.