  Radiative heat transfer between bodies is often dominated by a narrow
resonance in the transmission, e.~g. due to a cavity mode or a surface
excitation. However, this resonant character is not visible in the average heat
current. Here, we show that the noise spectrum of heat current can serve as a
direct probe of the heat-carrying excitations. Namely, the resonant mode
produces a sharp peak in the noise spectrum with a width related to the mode
lifetime. We demonstrate that heat transfer in realistic superconducting
circuits or between two-dimensional metals can realize our predictions.
