One test of fluency would be to have the person watch a modern film in the language of interest and ask them to transcribe the dialogue into text and also describe in own words (in language of interest) what is going on in the movie.
You can then put that person on a scale from 0 to 100 by comparing the score by a random sample of "known" native speakers.
This test is nice because it probes many aspects of language: listening, speaking, and writing (but not reading), which can vary greatly between people.
If you're interested in probing reading, then I suggest asking the person to explain meanings of songs or poems.