So, personally, I'd put rarely stuck for a word in a variety of situations (television, conversation, shopping etc.) as a pretty good indicator.
That's definitely my definition of fluency. Also, I think there is written and spoken fluency, some languages, like Japanese or Chinese, you can become fluent with the spoken language, but be lost when it comes to books or newspapers. Danish or French are the other way around, you can read university textbooks, but be confused when walking around the streets of Paris or Copenhagen.