Okay, I went back into the editor and put this range of high hills between the Germans and the bridge, then got the Russians to blow it on turn 1 again.
I then waited 5 turns to make sure the dust had gone, then took this screenshot.
The Germans can see the bridge is blown!
Makes sense to me, as in real life I'm sure an attacker would make recce flights (abstracted in CM) to get vital intelligence about which bridges are up or down before launching an attack..