What's the easiest way to find rooms I haven't gone to?

In an effort to get the Tourist achievement ("Visit every room in the game"), I went back through each area and made sure I walked into every room. Alas, after beating the game, my progress only appears to be 279/282.

Is there an easier way to find unvisited rooms than going to each stage and opening the map? Is it possible to see all maps without having to visit each location? Are there any rooms that aren't listed on a map?

You might try reading through the side quests at the end of the guide here. Certainly not ideal, but you may notice one that you don't remember doing, since you have so few rooms to go.