short between isolated circuits
The only way to detect whether the two circuits are "shorted" together from inside the circuits is if some current flows between the two. In order for that to be true, there has to be a return path for the current between the two in addition to the "short".
If the two circuits do not share anything in common, and are totally opto-isolated, there is no way to detect a single "shorted" point between the two from within either one.
An analogy would be to lock yourself in a windowless room and try to detect when someone is touching the wall on the outside of the house - you can't do it without some information from outside the room.