I want to put a condition in a script that checks if the player discovered a certain location (namely, one or more of the Standing Stones). It's not a quest script but rather a Magic Effect or maybe a script attached to the item. But I can't find any function that fits the bill in the CK. Is there some way I can do it?

Dang. Never you mind. http://www.creationkit.com/CanFastTravelToMarker_-_ObjectReference

Amazing how you find a solution five seconds after you ask the question. That should do the trick.