AOE for certain skills

Is there anyway to determine what the AOE is of certain spells and skills. I'm most curious about the range at which a Necro can benefit from those dying around them. Just how close do you need to be? Is spellcasting distance enough or are we talking about melee range? I was wondering because I picked up a couple of Mes skills that require enemies to be "nearby" to function and I would like to know just how "near" we are talking about. Sometimes its hard to tell if you're gaining the benefits since there isn't always a clear visual queue like with Firestorm and Favourable Winds.