I think that you are fluent when you
1 - realize that you are dreaming in your new language when you sleep.
2- Are able to make jokes, quips, puns and original clever remarks.
3- understand the sarcastic humor in the most recent movies, videos
You know when you are fluent when you have the "Scrubs" moments in another language. I'm talking about inner dialogue or narration. You basically start thinking about the situation in another language. And also when you're having difficulties to instantly switch into your mother tongue.