How to tell if someone is fake?

So a girl in one of my classes when we were in school… I guess I could call her a friend but there was something suspicious about her, first I followed her on Instagram but maybe a few days later she unfollowed me, then an incident where I asked her to bring special brownies one day becuz I've never tried them she forgot and kept forgetting and didn't bring them in the end, one last thing she doesn't seem to answer my texts like I will say hey girl or something we both have iPhones and it tells when she saw it she never replys. So what's her deal? Was she pretending to be my friend cuz we had a common friend in the class? (Also I didn't repeated txt her, expect about the brownies)

She's deff fake. A real friend wouldn't need continuous texting and would be happy to spend time with you, do things for you, etc.