During a raid recently it came to my attention that there are people who don’t know how to tell if a mob ability can be interrupted. I can’t recall exactly when Blizzard added in the ability to distinguish between abilities that can and cannot be interrupted, but here you go:

Example: Ability cannot be interrupted
As seen above, the shield around the ability icon indicates it cannot be interrupted. No shield means it can be interrupted.
That, I did not know.
There’s a standard UI now?
I only know because I’m sometimes asked to interrupt these days, which I never had to do on Tardin. I think it was added to the standard UI in either 3.0 or 3.1.
Of course this does nothing to help you know if they can be stunned, but at least it takes a bit out of the guess work.
Damn X-Perl! I don’t think it has an equivalent. I haven’t run native Blizz frames for 2 years or more.
I think I will start researching this a little more.