The folks at Bell Labs were asked to figure out if Mark Williams had copied Unix directly or via too much knowledge already obtained, or whether it was truly a clean room recreation. I don't remember all the details, but it became clear after a while that it was indeed a reasonably clean rewrite.
This was done by looking for corner cases that were an accident of the original implementation and would be unlikely to appear in a version created separately. One detail that did stick with me was the discovery during this process that ppt, the paper tape simulator, mispunched a letter, I think "R", but the Mark Williams version did not. Was that compelling? Not on its own, but it was funny and memorable.
-rob