How can I tell if a girl is just being friendly to me, polite, or she is actually interested in me? confused?

Anonymous
It's very confusing, there is this girl I have met and talked to in school recently, and they are receptive to me when it comes to talking and having conversations. They give me eye contact, smile, and they ask me questions back in return, like they engage me in the conversation. I do not know if they are single or taken already. Also there is this other girl who I have just seen around campus, and she has smiled at me and said Hi to me, and yet I have never even met her before or spoken to her yet. I don't know if she is just being friendly, or if she is interested in me. Anyway, how can I tell if a girl is just being friendly to me, or is actually interested in me, that she likes she me?
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I some people will say that if a girl smiles at you and says Hi to you out of nowhere, they say "grow a pair and go up and talk to her". Yeah I will, but I want to make sure how she perceives me first, I don't want to get teased or led on.
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my bad, spelling error in the last sentence, I meant "how can I tell if a girl is just being friendly to me, or is actually interested in me, that she likes me?"
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Do most guys ask a girl for her phone number even if they are not sure if the girl is just being friendly or when they are sure that the girl likes him? Is it the same way with asking a girl out? Do most guys just go for it even if they are not sure?
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Lastly, so what are the obvious signs in which a girl is just being polite, friendly? or if she actually likes the guy, is attracted to him, is waiting for him to ask her out or for her number.
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I would like more responses and answers please.
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I have a hard time reading a girl's non-verbal hints.
How can I tell if a girl is just being friendly to me, polite, or she is actually interested in me? confused?
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