Supermassive black holes reside at the centers of galaxies and likely play an important role in galaxy evolution via active galactic nuclei (AGN) feedback. However, our understanding of the role of AGN in galaxy evolution is limited by our incomplete census of AGN, especially in low mass galaxies (i.e., with stellar masses < 3 billion Msun). In order to improve our ability to identify AGN in both low and high mass galaxies, we investigated the efficacy of the traditional optical and infrared AGN diagnostics in a sample of >300,000 emission line galaxies from the SDSS. Combining the emission line measurements with the deepest available WISE photometry, we have jointly analyzed a variety of different diagnostics, and present new recommendations for using the traditional tools and a new diagnostic sensitive to the presence of AGN. We apply this diagnostic system to a sample of >500,000 galaxies that lack optical emission lines and briefly describe implications for future studies of galaxy and supermassive black hole co-evolution.