Combining observations of spectral lines with different Zeeman sensitivities, it is possible to infer some properties of the small-scale solar magnetic fields. This paper presents a theoretical analysis of the diagnostics potential of 14 different spectral lines when used for this purpose. These lines span a broad range of wavelengths from the visible to the near-infrared and have been employed in recent studies of the quiet-Sun magnetism. It is shown here that the visible lines analyzed provide only a rough picture of the unresolved field distribution, but their results are more robust. Infrared lines, on the other hand, have the potential of carrying much richer information on the details of the distribution. However, this depends critically on the sensitivity attained and the analysis performed. The best results are obtained when a suitable combination of visible and infrared lines is employed. The analysis of sample visible and infrared quiet-Sun profiles confirms this assessment and suggests the coexistence of weak and strong fields within the resolution element, which would explain the observational discrepancy that surrounds this issue. Finally, I present a table with optimum combinations of spectral lines for three present or upcoming multiline solar spectropolarimeters (THEMIS, SPINOR, and the TIP+POLIS combination).