Hi Pete, you cannot identify specifically but when sales are slow, feedback trickles in say one or two a day and you subscribe (free) to dsrwatch it informs you of even small changes by email.
I always used to check my stars before responding to feedback anyway, dsrwatch gives the value numerically to greater precison.
Of course if sales are good with a large quantity of feedback flowing in all day then the unreasonable ones should be swamped. So in that respect when you are most vulnerable it is the most effective.