> If a corporation is selling an optimized binary, then it can be almost impossible to prove that there was any violation of the GPL without viewing the source.

I think you can notice that output looks similar, error messages are similar, etc. If the program is non-trivial its usually pretty obvious if its a copy or a reimplementation.

If it sounds plausible, presumably you could sue and read the source in discovery (ianal, not sure precisely how that works)