怎樣知道小提琴的上漆是否上得好？

Smooth, evenly distributed, no bubble, no hollow, not too thick for shiny, not too thin to protect the wood underneath. These are the things you can observe through naked eyes.
You can compare two different prices of violins and you can see the differences.
However, this is only for comparing new violins. For choosing antique violins, more advanced knowledges and expriences are needed.