It seems that if you're looking to verify manufacturer claims, then maybe you need to look into how manufacturers rate their ropes. Does 500 lbs mean static tension, holding indefinitely? Or does it mean a one foot drop with 500 lbs attached?

It seems like anything involving more than a few hundred pounds could be very dangerous. I started thinking about how I'd design a tester, and then I thought, hey, someone's probably done that. Lo and behold:

http://www.youtube.com/watch?v=JVRI45AXOHA

It seems that the company (http://admet.com) sells a bunch of testing machines. Buying one of these is probably the best thing to do, though it may not be the cheapest.

If you can work with metal, you may be able to make a similar tester with gears.