when Razor invites Bill to tea...
They go into his office. There are little gadgets everywhere that he has been tinkering with, the decor a justaposition of old and new... That was the moment i was sure it was the Master. There are little hints sprinkled throughout:
The room is humming throughout the scenes in the office. They're in his Tardis!
Right after he makes the joke about the fast bottom and snickers the music shifts to part of the Master's theme (~45s in):
He's watching the Doctor explain for months with Bill.
He takes Bill outside on a field trip, as if she's his companion.
He gets his age compared to Bill's completely wrong (as the Doctor has done repeatedly).
He likes the hat and wants one when he gets upgraded.
He decides it's time to upgrade Bill as soon as the Doctor makes his way downstairs.
He's more interested in meeting Missy than the Doctor.
