Testing the integrity of a medial collateral ligament in a dog or cat is quite simply. Be sure to perform this test after doing a closed reduction of an elbow luxation or on any elbow trauma patient.
Use the contralateral elbow to compare to, assuming that it is normal.
Place the elbow and carpus in 90degree flexion.
Grasp the paw and externally rotate it
If the amount of rotation is excessive compared to the normal paw, then the medial collateral ligament is torn and surgical correction is required for a good to very good outcome.
To test the lateral collateral ligament, internally rotate the paw (instead of externally) to see if there is excess motion there. The lateral collateral is not damaged as frequently as the medial one but should still be checked.