How to know if she's one or not? Hmmm....
But crux of your intention, why do you want to know so?
If the girl has the intention of lying to you, she will die die say
she is one even she's not one. Just like a guy if he's one or not,
even if he's not, he'll say he is so to save 'face'.
But just take my case as reference, my girlfriend isn't one, but me
accept what she is cos me love her.