In the manual therapy video below, NAIOMT's Angela Gordon demonstrates a posterior shear test, which can be used to check for ligamentous instability.
The patient is asked to gently push into the table while the PT blocks the inferior vertebrae and feels for the superior vertebrae to slide backward. Over pressure can be applied to further assess ligamentous integrity. In a normal spine there should be little mobility here. You can compare various segments to determine where there is excessive mobility.