Current maximum Artifact power

[yoshimo]

I want to avoid showing a button to use Artifact power items when the character is capped and all that would happen after the cast is the red error "your artifact has maximum power".
The question is how to do this?
I could go for ranks purchased/learned from GetArtifactInfo, QuestCheck (maybe attack on the broken shore by Khadgar?), but i am unsure if this would be dynamic enough compared across classes.