How do you know if there’s an incorrect latch on?

Bub’s cheeks are dimpled or hollow; or bub is making a tongue clicking sound; or your breasts are feeling or looking misshapen; or your nipples are flattened or creased after feeding; or you are feeling real pain throughout the feeding — then these are all strong indicators that bub is poorly positioned (or part-way through a feed a good latch-on has deteriorated).