Crossover art. Make sure that no text or artwork is incorrectly cropped at the spine where pages meet. Make sure that nothing falls short of the spine if it’s intended to go all the way to the center of the spread.
Correct pagination. Refer to the folding dummy created by the print service provider’s planner or imposition department, and check the imposed blueline against the pagination in the original folding dummy.
Changes from page proof stage. Make sure nothing has moved or disappeared, especially if corrections have been made since an earlier stage.