This goes for attacks as well. Is that character standing in water that my ally is also standing in, or is he just enough out of it that air damage won't stun them both? Can be really hard to tell. I'm not sure what kind of indicator would work for that, without telling you exactly what is going to happen across the board with an attack (part of the skill and fun is paying enough attention to figure out how the battlefield is going to react to something.)

I guess there could be some kind of similar colored indicator at the character's feet that they're standing in an element.