Not sure ... but you can tell if they cast the spell by watching the progress of the spell on the section in the upper right corner. You see them build the towers, then underneath it is a bar that fills up as the spell progresses.
I hope that was clear, I'm not sure how to explain it better. But it's right underneath where the little icons of the towers show up. Once something appears there, you're pretty much screwed ... unless you've already cast the spell and yours is further along than theirs. Or you're surrounding their last city and can kill them before it completes.
