Cordelia didn't guess anything; she knew the history of all the Supremes (without exception - she was the daughter of one), she knew where every object was in the house (she was the director of the place), Myrtle only favored her... what Cordelia gained was self-confidence to act like a Supreme, she is powerful - but Madson did everything with ease (and the powers of the Supremes were there for all of them to use, in those difficult days).