First question: A good guitar player a steady rhythm and hit the appropriate notes with appropriate volume and tone. At a higher level, they improvise in a way that sounds good. Sounding good seems to involve sticking to a standard scale with only a few deviations, and varying the rhythms. At the level above that, I really don't know.

Second question: I really don't know, at least that generally. I think I may use proxies such as the ability to find novel (good) solutions to problems and draw on multiple domains, then aggregate them into one linear value that I call "intelligence". I am probably also influenced by the person's attractiveness and how close their solution is to the one I would have proposed. I would definitely like your take on this as well.