Know where the shoe pinches

Example: “After years as a nurse, she knows where the shoe pinches in this hospital.”

Origin: The image goes back to a story told by Plutarch: a Roman asked why he had put aside a wife nobody had a bad word for held up his shoe and answered that it looked sound enough to all of them, but only he knew where it pinched. Chaucer uses the same figure in the Merchant's Tale, and English has kept it since. The point is always that the trouble is invisible from outside and obvious to the one carrying it.