Answer: A doctor will not be able to tell that you masturbate. Masturbation does not change the appearance of the vulva in any significant way. Very slight changes in the size of erectile tissues may occur as a result of regular masturbation, but in ways no one will be able to notice. There is so much normal diversity no one can draw any conclusions about a woman's sexuality from looking at a woman's vulva.
Also keep in mind, 80-90% of women who visit the gynecologist have masturbated, and the gynecologist likely masturbates too.