How to differentiate a metal from its ion?
Other than the excellent answer by F'x, there is also actual spectroscopy, specifically UV/Vis spectroscopy. This'll only work if the metal has transitions in the right range, though -- transition metals, lanthanides and actinides are usually fine, s or p-block metals and metalloids, probably not.

Then you're looking for the absorption/transmission spectrum of the ion and the metal separately, and how much of each there is in a combined spectrum. It's not strictly quantitative, but you can get a good idea of the ratios.