Exploring the locations of monsters in a Minecraft map

If you're okay with using a mod, you can install zombe's modpack. There is a mod called "cheat" in there, and it has a feature that highlights all mobs through walls with a colored marker. Here's what that looks like in-game:

It works in multiplayer as well if you need that support.

If you don't like installing mods, you can always use MCEdit to explore your world, but development has become a bit stagnated, and you'll need to figure out how to compile a dev version to get it to work with MC 1.1.

As Jake King said, you could use zombe's modpack, but I think your asking for a different part of it:

Its the safe mod, and the red cross hatch area is where monsters can spawn.