Can doctor really find out if girls are a virgin?

Not always: This is not a discussion most doctors will have with others. The patient already knows.

Depends on situation: Sometimes this is quite obvious if they are treating a girl for a significant std. Otherwise, most find out by asking the question. Then there is trouble with the definition. Some relate this to a hymen that may not be present to begin with, not lost through sexual activity. I never assume, i just ask.