My thoughts on this are as follows:
Music isn't "of a standard" per se .... unless a composer is composing music to specifically target/improve/ practise a specific skill or set of skills as is the case with some who compose music specifically for instrument tutor books.
A composer will simply compose for the love of doing so and for the love of the end result. It may be on occasion (depending on the composer's method) that the standard of a composition may be limited to the composer's own level of playing. - For example a pianist who composes his/her own music by playing it.
However, a non player may compose something technically very difficult. I remember composing a simple clarinet melody for my GCSE many years ago. It would have been very easy to play on clarinet. I added a harmonising flute part. As I'm not a flute player I had to hope it was ok and checked with my sister that it didn't go too high/low etc. I later added other parts - percussion and piano.
I remember my teacher commenting that I'd written a really rather lovely piano accompaniment and that it was probably working in my favour that I wasn't a pianist as if I was I would never have written the rather tricky but lovely ornamented ending ....
I think that the simple answer to your question is that
a person's ability may be somewhat arbitrarily measured by their ability to play a certain piece of music and whilst doing so meet a certain set of criteria: observation of dynamics, correct articulation, expression etc etc (e.g. graded exams) but the music alone is simply music and cannot be said to be of a "set standard" unless written expressly for the purpose.
edit: grading for difficulty .... can give a buyer a guide to whether the music is suitable for you - so you don't find it far too hard or easy but bear in mind that whilst a challenge is always good for stretching us/improving our skills as players for performing it may be better to play something that is "easier" well than struggle with something "too challenging". Your audience will certainly enjoy it more.