When Irene Lanzer's ('Aleksa Palladino') feet are shown they are clearly too nice looking to belong to a ballet dancer in the midst of a ballet season. Dancing on point shoes causes many sores on a dancer's feet.
As the Prima ballerina exits Holmes' room after the insinuated coital matters, her feet are shown. The feet lack all the blisters and damage caused by the "pointe" shoes used by professional ballerinas, making her feet unlikely to belong to an active ballerina.