Nearby

When an Area of Effect skill affects nearby targets, this means that it affects an area of 20 feet around the target of the spell. (For comparison: The "aggro circle" measures 88 feet.)

For offensive spells (like Chain Lightning and Chaos Storm, this can be determined by the red dots of the enemy on the mini-map. If two enemy red dots do not touch, then they cannot be affected by the same AoE spell.