Even a simple thing like cycling can tear up the hymen. So intact hymen cannot be considered as a proof or virginity. yes a doctor can diagnose whether a girl is used to intercourse or not. Now a days people are not orthodox enough to expect bleeding on first night.Unless he is experienced, he may not be able to peitrate vagina in the first night. So take it easy. You may bleed if there is full penitration.
Most women have a reptured hymen long before they get married. You probably cut yourself from masterbation.