Nearby Mob Finder Mod 1.8, 1.7.10, 1.7.2 adds the ability to see the nearby mobs whenever you get the message: "You may not rest now, there are monsters nearby" It does this by posting the coordinates of the mobs and it's name in chat.
There is also small feature that allows you to check for the nearby mobs to your own character at any time. This is disabled by default in the config so make sure to enable it if you want to use it. The default key for this option is "U" but it can be changed in the options.