How do I know if she already has a boyfriend?

[Marco]

Hey Benjamin, Marco here,

Well, you have couple of options. Try to ask her friends, or ask your friends to ask her friends. You can also ask her indirectly: "Hey, nice necklace, from your boyfriend?". you can learn a lot from her answer. if she says "no, I don't have a boyfriend" she might be interested in you. "No" means she might not be interested in you and "yup" means she might have a boyfriend or might not be interested in you ;-)

After you figure out this boyfriend mystery, don't tell her you like her. What are you 12? Ask her out. Something casual, coffee, cake... You might even come up with something you want to talk to her. "Hey I heard your sister went to hardvard, would you have time to sit with me and talk about this because I am also thinking about going there."

If nothing works out, you can have a mutual friend set you up or use something like 2kuut.com to get things moving.

Hope I helped and good luck