Once I had a "pet" city of the Americans, their last one (Washington), completely surrounded by my cities. Each turn I'd send a spy in to just look at what they were up to. Their shield production was down to two per turn (the rest were used to support something like eight units in the city), BUT when they were building something like a cannon or engineer, EACH turn they'd contribute five or six shields more to the production. Also, when they were building SS structurals (they stole all the spaceship techs from me, which precipitated the war that brought them down to their condition), they would do these in two or three turns, WITHOUT spending any gold (I kept track of their treasury). Which peeved me because in that game, I was going for a max score, and wanted to launch my max-sized SS at the last possible year (2008) to get it there a year before forced retirement, all the while building metropolises all over the world.... But each turn my spy would look, that SS part would have like 40 or 50 more shields on its completion, with a city producing only two, no freights, and no money spent! So I had to launch in 1998 when they were close to being done with theirs (I would not break a treaty, and break up my democracy, to sabotage their production).
No worries, it was a high enough score (699%), and I won't play for max score anymore (TOO damned tedious, an hour for five or less turns at the end), but this is something I just wanted to share--the AI will cheat EVEN when you're watching them closely and catching them red-handed!