Given a system of massive quantum harmonic oscillators, how can we test whether their gravitational interaction is classical or quantum? Here we propose a general Gedankenexperiment to do so and benchmark it, i.e. we show how to determine the threshold beyond which one can claim that gravity behaves as a quantum interaction Hamiltonian instead of a classical field. The remarkable aspect of our approach is that it bypasses completely the problem of how a classical gravitational field may interact with the quantum oscillators. Using a delightfully simple trick from quantum information, we are able to give a bound that applies to all physically conceivable scenarios at once. We find that in a certain regime the best classical strategy introduces an error that grows linearly with time, with a coefficient that depends on the geometry of the system.