How can I tell which marvel sites I have investigated already?

I'm trying to wrap up the Visions in the Storm quest. I have been sending my explorers, soldiers, and pet sea dragon to various hydrocoral brains, but I have a hard time finding out which one I'm missing. (I had some of my units doing automated exploration, and now I'm not sure which one I might have not visited yet.) The only way I seem to be able to tell whether I had investigated one before is by revisiting it with a unit and checking whether I'm able to investigate it. This is consuming a lot of moves of my units of course. Is there a better indicator to tell which hydrocoral brains I have already investigated?