Crush kissed on the cheek? Is she taken?

Hey, i have crush on someone i study with in my university, i have seen her 5 times so far and in all of those she sat next to this guy. Last time when everyone went home i saw him kiss her on the cherk. Should i assume they are together or is it only a kiss on the cheek? The reason i am in doubt is that for the two times that i have spoken with her, she smilled to me a lot and also in the first time we talked, she told my goodbye when the lecture ended even thugh we only talked at the the beginning of the lecture for 4 minutes when i intredouced myself to her, and asked her about study.( altough of curse its possible she was just being polite) Thanks.
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Cheek**
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The thing is do want to ask her out soon, but i dont want to make her uncomfortable if she has a boyfriend, and the only real way to find out is ask her out, but should i ask first if she has a boyfriend, or just let her reply to the date offer to asnwer that question?
Crush kissed on the cheek? Is she taken?
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