Epitaxial growth continuously plays an indispensable role in both fundamental and applied research in nanotechnology. The detailed characterization of epitaxial growth provides essential knowledge to control the construction of nanostructures. A novel technique to characterize a surface will be demonstrated in this presentation. On monitoring the evolution of surface states with angle-resolved photoemission spectroscopy, not only the chemical properties associated with the surface states are explored, but also the chemical elements in the topmost layer of the nanostructures on which the surface states reside can be identified. The technique was applied to the epitaxial growth of Cu on Ag(111); our results indicate that the chemical element on top of the Cu islands on Ag(111) with the (9x9) reconstruction was not Cu but Ag.