What if multiple structures have the same number of production requirements? That aside, is there a way to get the game to tell you directly what other civs are working on a wonder? What is required for that?
Doing math, as if all the screen switching and so on wasn't enough of a pain and a time sink. Is there a mod that does this for you?
Then you're left with a bit of educated guesswork, but you can narrow it down based on what techs an AI has access to, what wonders have already gone, what wonders they're likely to build and so on. There is a way to directly see what an AI is building, but that requires city information, which requires you to have a huge amount of espionage points on them. Easier to do when they are spending their points to poison your waterholes.
Not that I'm aware of.
My point was the worthiness of espionage in the first place. Also it need not be espionage or research. I like going straight ahead on one path with research and some espionage for stealing tech left behind (as well as trading of course, good way to get trade tech this way). Especially if I can get say statue of liberty and representative and national park and fill everything up with spy specs. You said
Courthouse allows a spy, spies give so much more. Sure you only get 1 verses two scientists with library, but there is a limit to how many units you can afford to set aside for spec anyway. A spy gives 5 total resources, representative aside, a scientist gives 3. Further multiplication from the spy structures which more bonus, especially Scotland yard. And of course you could build both and or eventually another structure that allows you to set more spies.
A spy does give numerically more resources, but what you can do with those resources is more important than how many of it you get.

you can use on things like tech steals, but that's RNG dependant, and it can only get you techs that AIs already have. Clever use of scientists, especially when you've got early Rep, allows you to research and bulb your way to a tech advantage that the AIs just don't have, and usually that tech advantage is used to roll people over with superior military units. It's great if you can steal Engineering from an AI on the cheap, but only when that AI has Engineering (or is willing to trade it for something else you "redistribute"), meaning that you're going to need either Gunpowder units or inordinate amounts of siege to effectively war against that AI. Whereas if you tech and bulb your way to, say, Cavalry, and your closest neighbour is still rocking Longbows...well, that war is going to end significantly more poorly for them. Much more rapidly, at that.
@AcaMetis TruePurple seems really big on Espionage. With enough EP invested on a particular civ, you can get city visibility. I don't use this much..ha..but does city visibility show what the city is building at the time. I can't remember. Granted, you would need a lot of EPs to invest in all civs to see every civs' cities. But it might be one way of seeing if a wonder is in process. Granted this does not work for unknown civs.
Yeah, I gathered

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City visibility is different from city information. The former only removes fog of war from the city tile and I think the first, but not the second ring of the city's tiles. The latter allows you to inspect the city as if you owned it and see what the city is producing, what tiles/specialists it's working, etc. Great way to see how badly higher level AIs cheat at literally everything, because their builds will literally be listed as taking fewer hammers than it takes you to build the same thing. Also allows you to see an AIs slider positions, for what that's worth.