If you'd like to prove how much you have progressed on your chosen instrument, a wide range of exams are available from various examining boards. If you are in school you can also follow the GCSE and then A Level qualifications in music. Gone are the days when this was a daunting experience - examiners these days do their best to make you feel at ease, and one or two errors are not going to mean an automatic fail.
Another good thing is that music boards are not religiously devoted to classical music only these days - so if you are focusing on jazz or pop music, there are relevant syllabuses (or syllabi? - definitely not syllabub, which is a kind of dessert).
If you'd rather avoid any stress, you can of course just play for pleasure. You can self-measure your progress by attempting more and more complex and demanding pieces, and by recording your efforts.
If you do decide to go down the exam route, we can advise you on which board to choose for any given instrument/style. We can also provide a tutor who is experienced in GCSE preperation and A Level exams. Many of our tutors have gone through these exams themselves and have helped many students gain thier music GCSE or A Level. Whether its A level, GCSE, or a music grade, many of our tutors have a 100% pass rate for students taking exams.