For over a hundred years, the Gouy-Chapman and later Gouy-Chapman-Stern model have been the accepted model predicting the electric field and solvated ion distribution in the electrolyte near a charge solid surface. This model is widely used in aqueous chemistry, colloidal science, membrane physics, and environmental science. Surprisingly, however, no direct measurements of the electric field vs distance from the solid surface have been made to confirm the Gouy-Chapman model. Using optical second-harmonic generation in total internal reflection geometry, we have measured the Lorentz transform of the electric field vs distance and use that to confirm the shape and lengthscales of the Gouy-Chapman model. We find that the diffuse layer length matches the model prediction and that there is a breakdown at short distances consistent with the finite size of solvated ions at the interface. We are able to use this technique to characterize the diffuse layer at the self-assembled monolayer/solvent interface and thereby measure the effective thickness of monolayers and their charge in situ with their native solvation layers.