I'm not an expert on language testing, so I don't have any specific techniques to suggest. I would say, though, that there are cases when it is clearly not necessary to test the person's language's skills. When someone says they are bilingual but their claim is not based on criteria that can be easily checked, it may be appropriate to test that person's language skills.
I am not an expert on the various levels of language proficiency, but what I, as a lawyer, care about is that the person can fully understand everything I have said and everything I have written in my factum. That is my criterion. It may not be the same one a linguist would use, but that is what I expect.