I also have runner's knee. It was diagnosed with an MRI. I have never had it pop out. Like someone else described, it is an ache or sharp pain around/under/behind the kneecap. For me, it hurts when I am going uphill or up stairs - no problems going downhill. Since I don't EVER run, I call mine "hiker's knee."
If you do get diagnosed with it, the little "straps" help a lot. They are velcro or elastic bands that go around the bottom of the patella and hold it in place. I guess it holds the patella steady?? Good luck!