Rachel Berry was quite simply a 'Cheerio Nerd'. She knew all about the Cheerios; how they were run and what they did and how. But she was even more of a Cheerio nerd for one specific Cheerio. The head Cheerio herself, Quinn Fabray. She knew all her secrets, all her body signals and every tick of her facial muscles.
Now this wasn't due to stalking or anything but too simply. . . observing. She watched as Quinn moved down the hallway, never looking her way but to sneer, and having the crowds part for her. Rachel watched when she was supposed to be concentrating on FInn in Glee but was, instead, watching the Cheerio move and flow to the music. The diva also watched as Quinn undressed for her in Rachel's solitary room at night, so sensual and seductive. She watched her face late at night as she slept.
Rachel Berry learned these inside things by observing and noting. But no matter how many times Quinn caught her staring at school, where they had promised not to revel themselves, she couldn't think of anything else she would rather be than a Cheerio Nerd.