How to determine a safe red-line?

Assuming it was built well (which I'm pretty sure it was if it is still together at 500 miles), you should be fine to the limits of that cam. Usually, if it is going to break it will break very soon after the build.

Let me preface this with I have not used the KB 107's before, so I don't know their limits....but, if it were me, I would be totally comfortable up to 6500, but your cam will probably run out of steam around 6000....so I would take it to 6k and see what she does. Have fun with it. :D