When should I not approach a girl?

I ask this question because sometimes i see girls at my college campus, in the library for example, they’re working on assignments and have their headphones on. I feel like i’d be a burden if i walked up to them and talked to them. But I’ve seen people approach girls despite that, and it goes well. I’ve actually done it before too, but im not sure if it’ll work 100% of the time. So i guess my question is, can i approach girls despite them being a little busy? I was thinking maybe i could slide them a note with my socials on it. But what do you guys think?
When should I not approach a girl?
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