Either it's on the ROBLOX wiki and I can't find it, or I've looked all over and it's not there, but I'd like to know if there's a line or something that can tell if a Core GUI is visible or not.
The method you are look for is called GetCoreGuiEnabled
Documentation on this method can be found here: http://wiki.roblox.com/index.php?title=GetCoreGuiEnabled
Example:
game.StarterGui:GetCoreGuiEnabled(Enum.CoreGuiType.PlayerList)
http://wiki.roblox.com/index.php?title=SetCoreGuiEnabled_(Method)
SetCoreGuiEnabled