a) Listen to them playing.
Speaking as a guitar and piano player. I can do things on guitar and piano that are fairly easy, but look very impressive to someone who doesn't play the instrument. You actually need to play an instrument before you can judge how good someone is accurately.
(Obviously, it's pretty obvious if someone is distinctly bad. But distinguishing different levels of "good" is hard.)
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.)