Nocrit is a simple add-on that shows you in the chat window how much defense rating or resilience rating you need to get crit-immune against mobs three levels above yours (normally bosses)

If you are already immune to critical hits, it will most likely show negative numbers that tell you how much of each rating you can remove from your gear before becoming vulnerable to critical hits again.

Slash-Commands:

/nocrit : will print out the info in the default chat frame

Enjoy.