DrKev
Moderator
You can use a cheap ohm meter to detect a buzzing fret by probing the string (at the post is okay), and then touchng the other probe to the frets. When you find the buzzing fret, since the string is touching it, the circuit is complete and the resistance goes to zero.
If the knob is really stubborn, you can use two guitar picks to get it off. They need to be medium-heavy. Slide one under the knob, slide the other under the knob directly opposite, and pull up on the picks.
I use two teaspoons. Probably not as safe on the finish but with care it's very fast and easy.