Only your girlfriend will know if she is having an orgasm. Asking her is not reliable as you could say yes I had an orgasm or when I orgasm this this and this happens to my body but you have no way of knowing if she is telling the truth. One simple fact though is that many women are not able to have an orgasm via intercourse alone so why not worry less about the actual orgasm and concentrate more on giving your gf plenty of pleasure. Ask her what she likes and what really helps her get off.