Shape of the incisor teeth is often used; the occlusal surface is a long oval in young animals, but as the teeth wear, it becomes round and then triangular, with the apex toward the lingual side. Additionally, the angle of the incisors changes from nearly perpendicular to more or less parallel as the animal ages and the teeth wear.
The more useful signs are arranged chronologically in the following list:
References:Philippine Veterinary Drug Directory (PVET) 16th Edition. (2018). Medicomm Pacific, Inc.