Iv had a right knee replacement for 10 yrs. How do I know when it's wore out? The back of my knee sticks out far as well.

Get an xray. Wear and tear on a total knee replacement will typically present with clinical symptoms of pain and swelling. Xrays of the knee will show any possible concerns of wear. I would recommend an orthopaedic consult with a surgeon specializing in knee replacement surgery.
Dr exam & xray. An xray will show wear of your replacement. The bulge in the back of your knee could be loose liigaments or knee swelling. A careful examination would clarify the cause. Sometimes ligament loosening can be caused by wear.