They diagnose this by what is cranial drawer maneuver. Its my understanding that Xrays really won't show anything other than arthritis if she has any. Here's a decent explanation:

Overview

A normal dog's knee joint works like a hinge, keeping the animal's leg stable as it bends. When the knee has a ruptured cruciate ligament, the "hinge" becomes loose and no longer functions as effectively. This looseness prevents the knee from maintaining stability of motion. As a result, the dog suffers pain, inflammation, and eventually develops arthritis. This is a very common orthopedic disease in dogs. When a knee injury occurs suddenly—as it usually does among humans who suffer from injuries to the cruciate ligament due to a skiing, football, or other sports-related accident—the ligament will tear rapidly. But usually with dogs, the tearing is partial and occurs gradually, resulting in low-level lameness that may improve initially over time. However, progressive injuries can be quite damaging; because of the trauma to the ligament, the knee joint becomes inflamed, leading to arthritis, which only grows worse with continued weight bearing. In more severe chronic cases, the "shock absorbers" of the knee—quarter-moon shaped sections of cartilage called the medial and lateral menisci—tear or become crushed because of exposure to abnormal stress that occurs when the knee is loose. Animals with this condition often are severely lame and may not be able to put any weight on the affected leg. Both surgical and non-surgical methods for treatment are available. Generally, dogs have a good chance of recovering normal, or almost normal, movement after surgery, but the arthritis that has occurred already will not be reversible. Additionally, dogs that have ruptured the cruciate ligament on one side are more likely to tear the ligament in the other knee. Obesity is a major risk factor for this injury and certain breeds of dogs are more likely than others to rupture their ligaments.


http://www.westlakeanimalhospital.co...ture__can.html