I was doing a slow print and wondered if the filament was actually extruding and not doing filament grinding. If the extruder is filament grinding, then the toothed gear will spin, but the filament won't go anywhere. Thinking about this situation, I came up with this easy to do mod. Makes me wonder why I didn't think of it before. Remove the top cover of the extruder and then the inner cover held by a screw. Using a marker pen, dab dots around the perimeter of the smooth wheel. Then put the covers back in place. When the extruder is feeding filament to the hot-end, you will see both the toothed gear and smooth wheel move in tandem. The dots help you visualize the proper movement. If the extruder is grinding the filament, the toothed gear will turn, but the smooth wheel won't. A quick 4 second video can be downloaded to see the action.