Sometimes you can get lucky and find a "live from" thread with pictures of the nightly menus. Otherwise, the all menus will be available on board.
On our last cruise, I wanted to eat at Sabatini's on the same night as the Italian dinner in the MDR (sub one Italian meal for another!) I just called the DINE line once on board, asked which night the Italian dinner will be served, and made the Sabatini's reservation.