Thanks for writing in… There is no real way to know if a girl is a queer girl just by looking at her outfit. Sometimes straight allies wear rainbow things to show that they support the queer community. Sometimes straight folks wear things that might make others assume they are gay, when they would never identify themselves that way. And sometimes queer girls “look straight.”
If you’re interested in a girl, try being friends first. Ask her to hang out, maybe study together or go get something to eat after school. As you begin to trust each other more you’ll be able to talk more openly about yourselves and who you are attracted to. If you feel safe enough and comfortable doing it, you might even try wearing a rainbow something yourself and see if she responds or notices.
The surest way to know who someone might be interested in is for that person to tell you. So try for a friendship, and if she discloses that she’s only interested in guys, then at least you’ll have a new friend.
Best of luck,