Can an obgyn tell if a girl has ever had sex?

Maybe: In the case of rape, with evidence such as semen or a torn vagina it is pretty obvious. If the hymen is intact, then probably she has not had sex. Many women break the hymen without sex. If there is an std, then yes. Most of the time, the best we can do is an educated guess.

Can gyn exam tell : It is very difficult to tell by exam, exceot if the hymen is intact, than you can say than she did not have intercourse.But hymen can break withot having sex, so if you dont give that information that you are sexually active, we can not tell.Exceptions are when some one has sexually transmitted disease like gonorrhea, syphilis, pid etc, than there is no hiding.