Considering different skill levels (I mean it is unfair to say people who can't play the cello as well as Yo Yo Ma aren't cellists), I think there's a couple of ways you can say that one has truly learned an instrument. This is just my own thinking, others may consider it differently.
It can be one or more of these traits:
-can sight read (this is especially true for classical and jazz musicians)
-can improvise AND within the limits of the band/orchestra/group
-can execute all basic techniques at a moderate speed
-can tune and maintain said instrument short of actual repair or any adjustment that requires a professional, such as piano tuning
-can compose an original tune using the instrument
-can perform three songs from memory
-can communicate using the correct technical terms
-has performed with the instrument on at least 5 paid or paid for productions/shows (not in the context of being used as a prop or the subject of comedy, satire, and the like)
-has been hired to perform the instrument for recording
-can teach or impart practical musical/ instrumental knowledge to a non-player such that they are able to perform at the same level as the teacher, with continuous tuition