simple.. a girl isn't interested entirely w/ you if:
She doesnt give a damn thing about you, whether you exist or not.
she always finds ways to elude you or make alibis when you ask her out
she simply and practically wish you would just go and die!..
she cajoles and makes ill fated remarks or jokes, calling you mascot names and etc.
Plain truth: if she's really not interested w/ you.. then you really have to take her word for it. It's an eye-opener that she doesnt like you or even spend a minisecond w/ you.. Period
If she's just playing hard to get.. trust your gut instincts.. surely a girl would be gullible enough to trust and express or professing her feelings to you.. (if she's just playing hard to get that is..) eventually... it's only a matter of time.