Some debuffs, DoTs or damage-after-time skills are limited to one target at a time, with a clear tooltip. Examples include Daedric Curse (either morph) or Cripple (base skill and one morph only). But others seem to work that way without a tooltip indication (last I looked Elemental Drain seemed to be one).
Is there an accurate guide for figuring this out, other than by trial and error? (What made me think of this is Total Dark, which from its tooltip sounds like a powerful skill, but never seems to get recommended for PvE rotations.)
Similarly, it's not always clear which skills can or can't be stacked on the same target by multiple players. (Purifying Light?)