I think whenever you meet your own definition of good, like being able to read music, or play a certain song, or pick up songs by ear, or play really fast, etc, thats one definition of good.
Another is when someone who knows what they are talking about says your good, like another good guitarist, or a music teacher, like what theDirt said.
Oh yeah, and I felt I wasn't a beginner once I successfully made it into my first band, whcih is actually where I learned that I could do things I didn't know I could before, like solo.