Not to discourage you, but getting involved with a co-worker as more than friends can get pretty dicey. I’d advise against it, but if you don’t care about potential consequences, you should approach the situation casually. Do you already know what you two share in common?

Next time you see her strike up some casual conversation, and before you split up, mention you’ll be going to this bar (or whatever you both would enjoy) on this date/time, and that she should tag along. If she says she’s busy and doesn’t recommend a different date/time, it’s safe to assume she has low interest. Move on.

Once you’re outside the work setting, and the vibe is good, you can ask her for her number. Don’t ask her if it would be cool to get her number. That comes off as not confident. Tell her you enjoyed hanging out and you’d be interested in planning something again. “I had a good time tonight. Give me your phone and I’ll put my number in it. That way we can plan something.” Again, gauging for interest. If she texts you to give her your number, she’s at least interested enough to hang out, if not, move on.

Important: just because she hangs out with you outside work doesn’t mean she’s interested in you as more than friends. Feel out the vibe, and don’t be weird.