How To Know You're Fat is about labels, particularly "fat." Through social situations, the narrator develops an insecurity for being overweight. She frequently compares herself to her peers, saying "when you and your friends go out to eat and everybody leaves something on their plate... but you demolish your chicken fingers and curly fries." Ultimately, it isn't others that do the labeling. We label ourselves fat.
I admit that I can relate. Being overweight for girls my age, I've developed methods to hide the fact. I've worn dark clothes to give myself a slim appearance. I've skipped breakfast before. I've never worn a bikini before, only a one piece swimsuit. When someone calls me fat, I brush it off, saying "Yeah, I know. I need to lose weight sometime." In reality, I label myself "fat" too. I need to learn how to unlabel myself sometime.