We report indirect measurements of the surface temperature of iron oxide nanoparticles in an alternating magnetic field (AMF) through the temperature induced change in fluorescence of a thermoresponsive/fluorescent polymer consisting of poly(N-isopropylacrylamide) (pNIPAM) copolymerized with a fluorescent modified acrylamide (FMA) monomer with fluorescent intensity that increases as its surroundings change from hydrophilic to hydrophobic. When the particles are suspended in water and subjected to external heating, the fluorescence is observed to remain constant up to about 35 °C, above which temperature it increases. When the particles dissipate heat internally in an AMF, the fluorescence intensity increases immediately upon application of the AMF, even though the temperature (as measured by an immersed fiber-optic probe) is below 35 °C. The observed increase in fluorescence intensity indicates a change in the microenvironment of the FMA due to the transition of the pNIPAM from hydrophilic to hydrophobic. This in turn suggests that the nanoparticle surface temperature is above 35 °C and therefore higher than the temperature of the surrounding medium.