I don't know about Classics and NG but certainly in the 737-200 there were red lines on the upper surface of the left flaps which could be viewed from the cabin and they were marked in various settings.
On the other hand, it could be argued that to have the rotten luck to not only have a electrical failure requiring flight on standby power - but to simultaneously have a flap failure even though both engine driven hydraulic pumps are working, is to stretch credulity a little too far?