What is the ruling on a Muslim girl going to a non-Muslim female doctor to ascertain her virginity, based on her fiancé's request?

The uncovering of 'awrah (intimate parts) between non-spouses is forbidden. The 'awrah of a woman in front of another woman is from the navel to the knee, and this prohibition is more severe regarding the private parts. Scholars have made exceptions for necessity, such as medical treatment. What the questioner mentioned does not fall under the category of necessity that permits uncovering, and therefore, it is not permissible for her to do so, even if it is with the intention of marriage.