  Assessing whether a quantum state $\hat \rho$ is nonclassical
($\textit{i.e.}$, incompatible with a mixture of coherent states) is a
ubiquitous question in quantum optics, yet a nontrivial experimental task
because many nonclassicality witnesses are nonlinear in $\hat \rho$. In
particular, if we want to witness or measure the nonclassicality of a state by
evaluating its quadrature coherence scale, this $\textit{a priori}$ requires
full state tomography. Here, we provide an experimental procedure for directly
accessing this quantity with a simple linear interferometer involving two
replicas (independent and identical copies) of the state $\hat \rho$
supplemented with photon-number-resolving measurements. This finding, which we
interpret as an extension of the Hong-Ou-Mandel effect, illustrates the wide
applicability of the multicopy interferometric technique in order to circumvent
state tomography in quantum optics.
