Fair question, but difficult to answer in brief, I might try to do this later. For now let me answer with a couple of questions:
How can you tell if someone is playing a guitar well?
In general, can YOU tell the difference between someone doing things intelligently, and doing things unintelligently?
How can you tell if someone is playing a guitar well?
a) Listen to them playing.
b) Do they have concerts, CDs, fans, other symbols of "being a successful guitar player"? Do they write blogs or books about guitar playing? Do people write guitar-playing-related blogs and books about them?
The second option is less reliable and easier to fake, but it is an option that even a deaf person can use.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
(I plan to make these threads from now on. Downvote if you disapprove. If I miss one, feel free to do it yourself.)