We introduce a technique to characterize, in situ, the solid-solid interface formed when a rough metal surface is placed in contact with a smooth mica surface. The technique is based on the use of extended spectral analysis of multiple beam interferometry in conjunction with the surface forces apparatus (SFA). The intensity versus wavelength spectra of light transmitted through the rough metal/mica interface is measured and compared to that transmitted through a smooth metal/mica interface. Interference theory is used to characterize the rough interface based on differences in the measured spectra. Roughness on the order of nanometers can be resolved. The key to our technique is the creation of a perfectly smooth metal/mica interface that is in all respects, except for roughness, identical to the metal/mica interface created in the SFA. We accomplish this by thermal evaporation of a metal film directly onto mica. We demonstrate the effectiveness of the technique by characterizing the structure of silver/mica and gold/mica interfaces. We also show how the technique can be used to investigate the exchange of fluids within the contact region.