Great question, there's always more to learn but I have three scenarios I think are quite telling. One involves a noisy social setting, a dinner or party for example. If you are able to comfortably maintain conversations and catch all attempts at humour then you're doing very well.
The second is a more formal/professional setting that requires you to talk at length about a specific issue. It can be a doctor's appointment, a conversation at the bank, or a chat with a mechanic. After five to ten minutes you'll have a great idea just how fluent you are.
The final scenario involves your ability to react to completely random situations, how well are you able to comprehend and express yourself without any preparation whatsoever?
These are my keys to fluency, I wonder what everyone else thinks?