Using a new approach, the interference SHG anisotropy, it is possible to unambiguously evaluate the nonlinear susceptibility coefficients, which describe the isotropic and anisotropic response of a metal electrode. We report on a first application of this method to single crystalline silver electrodes in a 50 mM aqueous solution of NaClO4. The isotropic coefficient a shows the strongest dependence on the electrode potential. Theory predicts that the real part a(r) = Re(a) should have a minimum near the potential of zero charge. A normalized plot of a(r) vs. the surface charge density sigma shows a good agreement between experiment and theory in the range -20 mu C cm(-2) < sigma < 20 mu C cm(-2)