In this work, we propose a method to compute the poles of the linear approximation to a non-linear system, locally around a stable equilibrium point. The method uses periodic inputs, just like hermonic identification of linear systems. We show in passing that the class of Wiener-Hammerstein systems is but a small subset of the collection of all nonlinear control systems, locally around a hyperbolic equilibrium.For this we dwell on the exact local topological linearization of linear systems as developed in [6].