William Meyer:
You don't prove it. You assert it. And you document every move a clean room member makes. Then it's up to a claimant to prove otherwise. At least in the US.
I believe there are some experience with "clean room" approaches in the free software community, especially when it comes to Java implementations. Som information could probably be found on the websites for projects like Japhar (http://www.japhar.org/), Kaffe (http://www.kaffe.org/) and GNU Classpath (http://www.gnu.org/software/classpath/).
Consulting these projects on how they have dealt with "clean room", free software and diversing laws in different countries seems like a good idea to me.