We have a warrior that used to keep his challenging shout and whirlwind binds next to eachother. (le sigh) Every once in a while "especially on vashj" I would see the taunt animation come up and our dps warrior gets flopped.
What I started to do, was use AGunitframes for my raid frames. I then picked up the addon to make whoever had aggro on my current target flash red, and put all the tank raid frames in a row. Blinking red tank is who used taunt.. But of course you can just look at target of target
On trash pulls occasionally (ha) a mob will get loose - some1 pulls aggro or a sheep get loose or a (god forbid) multishot. Thats when the tanks go into overdrive to pick things up. I generally taunt when I see ToT not equal to Tank.
And, of course, also my insane curiosity really wants to know:
1. Wow! Did some1 just taunt that trash of me?
2. If so who?
3. Forty lashes with a wet noodle for you!
On bosses such as Al'ar also its just good to know.