Quora asked this question recently: “How do you know when you are fluent in a language?” here
I answered thus: My guess is usually you won’t really know. Native speakers are usually polite and will flatter you with a better assessment than is true. Maybe a better question to ask yourself is “When am I fluent enough?”
My guess? When you’re enjoying using it and not really thinking about it. I am fluent enough in Afrikaans and can happily hold any conversation with someone who only speaks that language. But even though I have spoken it since I was little, no native speaker would mistake me for a native Afrikaans speaker.
Confession: I laboured under the mistaken impression that I was completely fluent. No-one told me otherwise. Then at age fourteen I went to Namibia (South West Africa as it was) and visited third cousins I had never met before. Within two sentences one of them blurted out “Jis! Jy kan hoor jy’s ’n rooinek!” (Boy, You can hear you’re English-speaking). And my bubble burst. I’m now amazed I was so deluded!
Another case in point: My 94-yr old Dad speaks “fluent Italian” which he learnt in Italy in WW2. I asked an Italian schoolfriend a few years ago “How well does the old man actually speak?” and he said “Really well. Really”. Somehow I think that’s politeness (two years in Italy seventy years ago when he was already 22yrs-old – ??). But I have no way of telling, so I’m happy to go with Claudio’s assessment! Thanks, figlio!
Another: I often get complimented for speaking good Zulu. This is definitely not true and is just polite people’s way of saying “Thank you for speaking isiZulu to me”.