Some aspects of this disclosure provide strategies, methods, and reagents for determining nuclease target site preferences and specificity of site-specific endonucleases. Some methods provided herein utilize a novel “one-cut” strategy for screening a library of concatemers comprising repeat units of candidate nuclease target sites and constant insert regions to identify library members that can been cut by a nuclease of interest via sequencing of an intact target site adjacent and identical to a cut target site. Some aspects of this disclosure provide strategies, methods, and reagents for selecting a site-specific endonuclease based on determining its target site preferences and specificity. Methods and reagents for determining target site preference and specificity are also provided.