How can I tell if she is truly interested?

Dorian_Grey
I work in a building where there are only 2 companies. The majority of the building is occupied by my company, but on the very top floor of the building, there is another company. There is this girl who works on the top floor (at the other company) and I think she is digging me. I say this because whenever I see her outside (we have a restaurant on the lobby and I eat my lunch outside), I always catch her looking at me. I haven't had a chance to chat her up yet as we don't work in the same company, and my badge does not give me access to the top floor of our building. Any tips for me? I am hoping to see her on the elevator one of these days and try chatting her up, but I'm also thinking that the next time I catch her outside during lunch, to possibly approach her then? I don't know what I would say to her since we don't work together, and I don't want to come off as creepy or weird.
How can I tell if she is truly interested?
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