Depends on the relationship machan. Some chicks just enjoy the company (time passing) and need a guy to be there when they need. But this doesn't meant that she will marry you. They are just passing their time till a better person comes by. If you ask her out she might just reject and tell that she saw you just as a friend, let's stay like that bla bla bla In this case this will fuck your mind up since it is not what you were expecting. But then again it is much better to know where you stand rather than just invest your time on a lost cause.
So just ask her out in some romantic way and get it over with. If she says YES you will be happy, but if she says NO you will cry like a bitch and then recover and be stronger.
Good luck putha.