Significance Amorphous solids are continuum media. However, their mechanical and thermodynamical properties, even though universal, dramatically deviate from those in crystalline solids. Their anomalous behavior reflects peculiar and universal deviations from Debye’s law in the low-frequency sector of the density of states D ( ω ) . Theoretical models predict a population of non-Goldstone modes following the universal power law D ( ω ) ∼ ω 4 that are subdominant and therefore hard to detect. In this work, we introduce a general protocol that can be used in both numerical simulations and experiments, to probe the non-Debye portion of the spectrum.