Plan of action:

Palpation under sedation should be the first step to check for ligament or 
tendons abnormalities.

Craniocaudal and mediolateral radiographs should be taken to detect bone 
abnormalities 
associated with the stifle.  (such as a previous fracture)

Arthrocentesis and synovial fluid analysis should be done to rule out infection.

Ultrasound and arthroscopy should be done as needed to rule out cranial cruciate 
ligament rupture if palpation under sedation showed a positive cranial drawer.