Homogenization of metamaterials based on the analysis of their reflection and transmission spectra often leads to nonphysical values of the effective permittivity and permeability. Here we address this challenge by analyzing the scattering spectra in terms of perfectly emitting and absorbing modes considering nonlocal constitutive relations. The decomposition of the scattering spectra into even and odd modes permits us to quantify the nonlocal contributions and provides explicit formulas for the effective permittivity and permeability which always satisfy the passivity and causality constraints.