Maybe its an option in the espionage menu - Find out all you can about [civ name]!!!
They then report back to you with full details on how many resources the enemy has, who they are trading them with, and who they are receiving resources from... At least i hope thats something you can do
So put this down as a Maybe
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I'm building a wagon! On some other part of the internets, obviously (but not that other site).