  Mid-infrared diagnostics are presented for a large portion of the Spitzer
Infrared Nearby Galaxies Survey (SINGS) sample plus archival data from the
Infrared Space Observatory and the Spitzer Space Telescope. Our main result is
that these mid-infrared diagnostics effectively constrain a target's dominant
power source. The combination of a high ionization line index and PAH strength
serves as an efficient discriminant between AGN and star-forming nuclei,
confirming progress made with ISO spectroscopy on starbursting and
ultraluminous infrared galaxies. The sensitivity of Spitzer allows us to probe
fainter nuclei and star-forming regions within galaxy disks. We find that both
star-forming nuclei and extranuclear regions stand apart from nuclei that are
powered by Seyfert or LINER activity. In fact, we identify areas within four
diagnostic diagrams containing >90% Seyfert/LINER nuclei or >90% HII
regions/HII nuclei. We also find that, compared to starbursting nuclei,
extranuclear regions typically separate even further from AGN, especially for
low-metallicity extranuclear environments. In addition, instead of the
traditional mid-infrared approach to differentiating between AGN and
star-forming sources that utilizes relatively weak high-ionization lines, we
show that strong low-ionization cooling lines of X-ray dominated regions like
[SiII] 34.82 micron can alternatively be used as excellent discrimants.
Finally, the typical target in this sample shows relatively modest interstellar
electron density (~400 cm^{-3}) and obscuration (A_V ~ 1.0 mag for a foreground
screen), consistent with a lack of dense clumps of highly obscured gas and dust
residing in the emitting regions. [Abridged]
