After they finish having sex inspect the men's ears to see if the small red dot really disappears.
If the red dot does not go away then you have to see if the women weighs more than a duck. If she does weigh more than a duck then she's a witch and therefore the men are virgins.
I don't know about the duck though.
There'll be no shortage of male "volunteers" to prove their virginity . . . . or the lack thereof. But, after awhile, isn't this going to take on the appearance of a house of prostitution??
(I'm just askin')
I thought that it proves that the woman is made out of wood, because if the duck floats and wood floats then they must be made out of the same thing.