A death is worth a thousand explanations. Basically, if you see the boss hitting you with a very powerful-looking attack, and you lost more than 50% of your HP, that was probably a tank buster, and you should have used a cooldown. You might not always die because of it, but it helps your healers because it means they won't have to heal back as much HP.

And if you happen to have ended up dying, well next time you'll be ready. :P

Some tank busters will have a cast bar that shows you're about to get hit. Others don't - these are the ones you'll have to pretty much predict when they're coming. It's all about timing and practice with these ones.

But hey, don't worry too much about it if you die to a tank buster. People make mistakes. Not to mention healers might forget about it too and forget to heal you back to full health before it, and you'll end up dying anyway.