Thermal transport in highly anharmonic, amorphous, or alloyed materials often deviates from the predictions of conventional phonon-based models. First-principles approaches have introduced a coherent contribution to account for these deviations and to explain ultralow lattice thermal conductivity, but direct experimental evidence for this mechanism remains elusive. Here, we propose that the frequency-dependent lattice thermal conductivity, $\kappa \left(\nu \right)$, provides a direct signature of coherent transport. Specifically, we show that peaks in $\kappa \left(\nu \right)$ arise from the frequency nesting of modes with identical wave vectors. Applying this approach to CuCl, we identify clear signatures of coherent transport in its dynamical lattice thermal conductivity. We revisit the interpretation of thermoreflectance experiments and argue that the conventional understanding breaks down in strongly anharmonic crystals, alloys, and amorphous materials. Finally, we discuss experimental pathways to measure $\kappa \left(\nu \right)$, offering a new route to verify coherent contributions in thermal transport.

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• Anharmonic lattice dynamics
• Lattice thermal conductivity
• Phonons
• Density functional calculations