What type of doctor? A gynecologist will probably be able to tell - this is how they gather evidence in cases of rape or molestation. It also depends on the situation - was the girl previously a virgin? (Was this her first time?) Was the sex violent or consensual? How long ago did it happen, and has she showered or bathed since then?
They can tell through DNA testing, if semen is left inside, but apart from that, there is no way of knowing just by looking. The thin layer of skin covering the vagina, the hymen, can be torn through first time penetration, but it can also be torn through using tampons, so you can't tell off that.