I have to honestly ask: What's qualifying Desecration Demon's second ability as a detrimental trigger? The fact it can be tapped down (and backfire on you) detrimentally by your opponents? I'd see the sacrifice side of it as gravy personally, but gameplay use doesn't apply as far as rulings are concerned.

There's enough stuff able to be Flashed in though so the trigger is still a very real thing. Snapcaster, Advent token, Restoration Angel, etc.

And on the “Pass turn” “Okay.” “Judge! My opponent didn't announce his detrimental trigger!” scenario, that seems real seedy and I'd be far too hesitant to issue a Warning for that. There are a LOT of players that aren't aware that if you're saying “Pass turn” or an equivalent you're just passing prioirity through the rest of the turn meaning the combat phase does occur. What would I do in such a case?

Edited Todd Dalton (Aug. 11, 2013 01:16:29 PM)

it's an ability that only your opponent can use, at his or her option.
literally every such ability is automatically detrimental.

As a more general method, you can also ask yourself if a vanilla 6/6 flying for 2BB would be better or worse than Desecration Demon. Since the answer is “significantly better,” the trigger is detrimental.

(You have to be a bit careful about this, especially if you have never been a serious tournament player and/or your card evaluation skills are unreliable. It works well enough in most cases though.)

A handy reference is the Missed Trigger Guide, in our wiki.