If you can find out if she is gay, then that would be best. You could ask her out without knowing, but given that she is a colleague, then not only do you risk the embarrassment of rejection, but you also risk creating an awkward/uncomfortable work environment.
I'd recommend that you start having friendly conversations with her. Invite her to coffee or lunch just as coworkers, not as a date. Get to know her, what her interests are, and how she spends her time. She might tell you if she already has a significant other or what her sexual orientation is.
Once you become friendly with her, you can friend her on social media (assuming she has an account on a social media network). Look at her profile and see what she has liked. Maybe she has a lot of gay-oriented likes, which wouldn't necessarily mean she's gay, but you can weigh that with the other information you found and determine if you still want to ask her out.
I was in a similar situation recently. A woman started working at my company and I had some friendly conversations with her and went to lunch with her. I was interested in dating her, but I was hesitant to ask her out because dating a coworker can be a tricky situation.
I friended her on Facebook and noticed some photos of her participating in a pride parade. At another lunch she talked about the pride parade, and although that didn't necessarily mean she was gay, it did make me wonder. Not long after that, I saw a post on Facebook from another woman who tagged her in an announcement that they were in a relationship!
So she is gay, and we can continue to be friends. If I had asked her out before though, then that could have made things awkward between us.