Empyrean magic is triggered by fire, light or healing spells. I have divine wrath (20 second cooldown), mass cure moderate (9 sec cooldown), cocoon (12 sec cooldown). The stack duration is 10 seconds.
For me the test is whether without any effort I am at max stacks during big fights. I find I always am because in difficult content one of those 3 spells is going to go off every 10 seconds. In easy quests probably not - but twists are for more difficult content in my opinion. It's possible other things (golem heart maybe) is triggering the empyrean magic besides what i have listed, but I am always at max stacks in the big difficult fights. I am not sure about judgement - I will have to test that. I throw those out quite a bit in ee amarath to setup chain reactions.
Things like lantern ring and eld blast don't trigger empyrean magic. i really like the extra 20 spell power and 10% critical chance - especially with warlock critical damage bonuses.
On the other hand blood radiance is always tempting but whenever I check I can't keep my stacks up.