I'm pretty sure this identity of the Grand Inquisitor had been known for some time, though I don't know from where, but the latest episode of Rebels confirm that
The Grand Inquisitor was a Temple Guard of the Jedi Temple and, as such, was present during the attack led by Anakin Skywalker, now Darth Vader, and the 501st Legion. He surrendered and was turned to the dark side, eventually becoming the Grand Inquisitor.
It seems highly likely that
surrendered Jedi would have been turned over to Darth Vader, who was not yet constrained to the black suit he later wore. As such, it seems that any surrendered Jedi such as the Grand Inquisitor who eventually survived and joined the Empire under the dark side might have known who Darth Vader really is, even though he would eventually wear a suit. After all, the name is the same as that fallen Jedi who led the attack on the Temple.
So the question:
Can it be confirmed whether fallen Jedi such as the Grand Inquisitor, who were present at the Jedi Temple during the attack, would know who Darth Vader really is?