As I noted above, the best I can do with range is that I can only get the range of an enemy targeting the enemy(This doesn't apply to friendly players in parties of course), which will be useless since I have the target and focus only option.
Also, I am busy implementing along with the target and focus only option, if enemies cast spells on you, you get the alert, but it has to be fully casted on you (for example, hammer of justice, or if you get feared)
You cannot know your enemies target unless you are targeting or focusing them.