How can I tell if I need to have a PCL repair?

Depends on symptoms. The best way to determine if you need PCL surgery is by your symptoms and what your knee feels like on physical exam. Mri or xray findings can show the injury but really how unstable the knee is during activities is really the most important thing.
MRI and exam. You should see a knee specialist after your primary care dr orders an mri. He can check for posterior laxity associated with PCL tears. The posterolateral corner of the knee may be injured as well.
Yes. Most PCL tears do not need to be repaired. If you are having instability issues, especially buckling when going up and down stairs or inclines, you should first try a rehab program aimed at quadriceps strengthening. If the knee continues to feel unstable, then surgery is something to be considered.