Keeping on top of things Punishment

How do I know when my Sub feels that she should be punished. Hell what does my sub constitute as a punishable offence? I do not want to wail away on her anytime the mood strikes me, but what does one set rules about. I have seen doms form their opinion on a multitude of thing, but in some cases it is because they are trying to set the mood.

I do not think I need to use punishment to set the mood for everything. There are times when she may want to go totally A-typical sub I guess and then I can see the whole eyes down approach, but for every day life I cannot figure out what is going to work.