Hi, how can I know if a girl had sex before marriage? I had sex wtih her many times but don't know if she had sex before me with some one else or not?

Hard to know. If you focus on building a long term relationship and you dont have sex immediately and she is content with that type of relationship then likely she has had fewer partners. Get tested for sti's and ask her to do so too before sex and each share your results with the other before sex, then you know you are much more likely to be in a safe and committed relationship.
You really can't! I believe it is important to know, based on mutual views, personal hygiene, potential for sexually transmitted diseases, and shared values. But an intact hymen may or may not be present in a "virgin." the issues of age and how quickly you and her had sex are probably the most tell tale signs. Please do not make this a hang-up!
Difficult. Can be very difficult. It's to late since you have been having sex with her. Best solution is ask her.
Difficult. Unless she a sexually transmitted disease, ie genital herpes, and you don't have it there may be a clue. Most people have had more than one partner. It seems very important for you and i hope your not placing extra strain on the relationship as a result. Would it change the relationship if she has had another partner? If it would, you may need to figure out why this is so important. Good luck.