Shy girls, how can guys know the difference between "not interested" and "hard to get"?

Both behaviors are pretty similar, specially with a shy girl. I have known this girl for a long time and she has never had a boyfriend. Now it is very noticeable that she acts differently around me, she is quieter, very sarcastic towards me and likes to tease me a lot. I really can't tell if it is because she just doesn't like me or she is hiding something. She is pretty chatty and funny with 99.999999% of people, I'm not going to say outgoing, but she will try to have a conversation or laugh at jokes here and there. With me she won't really initiate any conversations, she usually just throws sarcastic comments at me when I'm talking to others and things like that to"get my attention" when I'm not talking to her.

When she does have my attention, she keeps telling me how she doesn't find me amusing, how much I bug her etc. When I say things like "im gonna go" or something, she starts "oh thank god you will leave me alone now". Its like a night and day difference with me. As much as we "dislike" talking to each other, we both remember conversations and specific phrases and words and things we have done, things that happened months ago. I know I remember because I like her and those things get imprinted in my memory. SHe says she never listens to me or care about what I say and yet she remembers even more stuff than I do. I know girls have a great memory but even girls who are my friends don't remember every single word I say and "try to catch me in a lie".

I figure she either likes me or really hates me because when she talks to others, it feels like they are her friends and with me she will talk but its very different, the things we talk about and argue about are more like the things people who spend a lot of time together and know each others' ins and outs argue about, so I don't know what to think, any ideas?
Shy girls, how can guys know the difference between "not interested" and "hard to get"?
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