However, there are some cases where it is reasonable to call a root fractured without seeing the fracture. Here is an example.
This 80 year old patient had this root canal done many years ago. She presented today with redness and swelling/sinus tract on the buccal surface. Examination finds #4 with normal pocket depths, lateral and apical radiolucency & class II mobility.
All of these clinical findings (normal probings, lateral lucency, mobility, sinus tract at the midroot) point to the probability of a root fracture at the level of the post rather than an endodontic abscess or periodontal abscess.
I recommended extraction due to fractured root.