How can I tell if my girlfriend is cheating?

How can I tell if my girlfriend is cheating or likes someone else? We've been dating for 4 months, and she says she loves me and everything. The other day she said she missed me since we haven't hung out in a while, but when we're in school she never tries to sit by me, talk to me , nothing. She rarely acknowledges me.
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So why would she say that stuff? Tonight is the bonfire for homecoming, and we're the sophomore attendants but she never even asked me to hang out tonight. Inoted I'm hanging with another girl, and i told her and it feels like she got more upset.
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The thing is, I don't really try to get her to acknowledge me, she just never says hi or anything in the hallway either way.
How can I tell if my girlfriend is cheating?
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