As I said, tests for that exist. I think you're confusing perfect bilingualism with the ability to understand.
When we think of someone who is perfectly bilingual, we tend to think of someone who speaks both languages without an accent. Oftentimes, they have one anglophone parent and one francophone parent. The kind of person who comes to mind is quite clear when we think of someone who is perfectly bilingual.
Supreme Court judges do not have to be perfectly bilingual in that sense. The role of judges is not to give speeches or speak to the media. That is why I don't think it's necessary to evaluate their ability to speak the other language. It goes without saying that, if someone is able to fully understand a legal argument, chances are pretty good that they will also be able to converse reasonably proficiently in both languages. I don't think the purpose should be to test the person's ability to write or give a television interview in English or French.