Bite marks

Close-up of underside of a Norway rat head, showing front incisor teeth.
Characteristic sign is pairs of incisor teeth marks or holes that are about 2 mm wide.
Photograph: By Muhammad Mahdi Karim (Own work) [CC BY-SA 2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons