I was wondering if it was at all possible when hovering a ground effect over a location a counter of people being affected is shown. E.g. I hover my Rain of Fire over a spot and it tells me how many enemies I'd be hitting if I cast it right then. I experimented a bit with weak auras with no luck, and I couldn't find any addons either.
I don't think there is a way to do this. The circle, I believe, is client based and doesn't give out any information about targets etc. Not until you actually finish casting will it provide any information.