I "know english" but I still need to consult a thesaurus or dictionary from time to time. I believe there are standard tests you can work through to determine

I "know english" but I still need to consult a thesaurus or dictionary from time to time. I believe there are standard tests you can work through to determine your level of fluency (at least as far as that standard is concerned). But that isn't really necessary. If you know that you are consulting the C99 standard for "obscure" specifications then it's safe to say you know the language. If you need an official standard, they are there.