  Optical diagnostic diagrams are frequently ambiguous as a test of the
photoionization or fast shock models of the narrow line regions of active
galaxies. Here, we present a set of UV line ratio diagrams which can
discriminate between pure shock and photoionization modes of excitation, and to
some extent, also discriminate shocks with ionized precursors from
photoionization. These diagrams use relatively bright emission lines and
reddening insensitive ratios and provide a practical observational test for
separating the excitation mechanisms of the narrow line regions of active
galaxies. The most useful diagrams are those involving the various ionization
stages of Carbon, [OIII]5007/H-beta vs. CIV 1550/ HeII 1640 and the purely UV
ratio pair CII] 2326 / CIII] 1909 vs. CIV 1550 / CIII]909. Temperature
sensitive FUV lines CIII 977 and NIII 991 also provide good discriminants. The
models are compared to observations of nearby AGN, and also to high redshift
objects where the UV lines are shifted into the optical.
