There's this girl in my class (we've known each other for about 2-3 years), we are both 17. She's really shy, and i don't wanna make a move on her if i don't know if she likes me or she's just being friendly. I know for sure that she has a huge crush on somebody, in my school or even my class ,but i don't know who. I assume that it's me because she speaks with no other guy, but we're not like bff's, we only share jokes, not problems.. etc.
She asks me to go places with her, although it's not just with her, but with some of her girlfriends. Once she asked to go to a museum with her and her bff and her bff's mother (idk), but when i canceled it because she asked me on a very short notice, she said it's ok, and she didn't want to go anymore as well. Did she mention somebody else coming just to not make me feel uncomfortable, because she thinks she likes me and i don't, because i haven't given alot of signs to her, to not freak her out.

How can i find out or tell that she's interested and not just friendly?

From my experience, if you know a girl for a longer time than say a month (depending how often you see each other) then you are usually just a friend.

Even if a girl feels attraction for you in the begining, it can wear out really fast if you don't make your move! Because not making a move means not that you are not interested in her, but that you don't have the courage of a "real man" to say what you really desire.
If you hesitate because you are not "sure" - she loses her attraction for you. As a man you have to step up and fight for what you want even if there is a real chance that you will be rejected.

I usually say a girl I like her clearly after at most a few hours after I first met her.

That way I have nothing to lose, I don't feel needy and I don't need to play any "games". And I actually feel very good after saying that regardless of the outcome. If the girl was attracted to me, I will get her with 99% certainty. And if she wasn't, this may actually turn the tables in my favor in many instances. And if not, well I just move on.

If I'm already in love with the girl before I make my statement/move, chances of getting here are almost zero..

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Life is either a daring adventure or nothing.

uf only i had read your advice a month ago :L