mike - why and what do you think you could possibly learn about watching someone write a character?
Not Mike, but I'll offer a thought. The brush is basically an extension of your arm. In the hands of an expert, the brush is an extension of their mind and body. Watching an "expert" brush a character, a trained person can discern how much arm or mind/body connection is involved.
From there, the trained person could also gauge how good the "expert" might be at wielding a sword, because the principles of body usage are the same.
It's the same reasoning Ueshiba gave some dancer a 10th dan in Aikido after merely watching a performance.