Scanning transmission electron microscopy in combination with electron energy-loss spectroscopy is a powerful tool for the spatial and spectral characterization of the plasmonic modes of lithographically defined photonic meta-atoms. As an example, we present a size dependence study of the resonance energies of the plasmonic modes of a series of isolated split-ring resonators. Furthermore, we show that the comparison of the plasmonic maps of a split-ring resonator and the corresponding complementary split-ring resonator allows a direct visualization of Babinet’s principle. Our experiments are in good agreement with numerical calculations based on a discontinuous Galerkin time-domain approach.