I am trying to "debug" my large monster spawning room. I am aware that all surrounding caves must be lit, but my trap still seems to wind down after a few minutes. I would ideally like to use a tool to figure out where the monsters are so that I can light up the remaining areas. Does such a tool exist?
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.