Do StarCraft AIs (the ones that are included with the game) cheat? (e.g. can they see past the fog of war?)
The ones created using BWAPI cannot (you can call a function that lets you, but it's banned in all tournaments). The one thing that BWAPI bots know that a human doesn't is a persistent unit ID - if a marine leaves fog of war and comes back, you can check if the ID is the same and know whether it's the same marine. It also tells you where cloaked units are, but a skilled eye can see those already.
I don't know much about the SC2 AIs. You could play some games and watch the replays to tell if consistently looks like it's responding to things it shouldn't know. I remember it being pretty blatant in BW.