Good question. I looked through literature to see whether anyone had come up with any good ways of measuring fluency, and there are a couple out there.

The key indicators that kept coming up were the number of unique words used and the speed at which you speak, so they're involved. I'm hesitant to give the full equation, because I thought it might be better to leave some mystery so we aren't consciously trying to game it while we learn.

It is by no means perfect, but I think it's useful to have some objective metric to track progress.