I would say that fluency is not about what you can. For example, if you can understand 70% of a movie, you most probably can get the rest 30% without too much trouble, just from the context. Or, for another example, you still can be understood even if your grammar knowledge is absolutely atrocious. So, understanding or measuring are quite relative things when it comes to the language learning.
For me personally, being fluent is just being confortable in a language, eg. to use a language (to speak, read, listen) for a long time without getting tired of it.