We present new methods, and improved calibrations of existing methods, to determine metallicities of late K and M dwarfs from moderate resolution (1300 < R < 2000) visible and infrared spectra. We select a sample of 112 wide binary systems that contain a late-type companion to a solar-type primary star. Our sample includes 62 primary stars with previously published metallicities, as well as 50 stars with metallicities determined from our own observations. We use our sample to empirically determine which features in the spectrum of the companion are best correlated with the metallicity of the primary. Lastly, we use our sample of wide binaries to test and refine existing calibrations to determine M dwarf metallicities. We find that the zeta parameter, which measures the ratio of TiO can CaH bands, is correlated with Fe/H for super-solar metallicities, and zeta does not always correctly identify metal-poor M dwarfs.