This is a very simple addon that remembers when NPCs are immune to CC effects (stuns, roots, ...) even though they haven't been CCed before, and adds a line to their tooltips.

I also included a list of CC immune mobs that I have encountered in current dungeons and raids and I'll continue updating it, so you don't have to start from scratch. If you want to help extend this list then please post the contents of your WTF/Account/<Account-Name>/SavedVariables/CantTouchThis.lua here in the comments, preferably in a spoiler tag.