Be confident! Ask if she wants to get a drink after work. Just a drink. You must convince yourself of this first. That way you will br more relaxed regardless of her answer. You will find out about the boyfriend situation that way.
If she smiles at you seemingly all the time, twirls her hair with her fingers, or leans toward you as you talk, these are definitely green-light signals that she is attracted to you. Find these or other friendly body-language signs. These help with the confidence thing as well.
Don't try to be overly cool, or overly anything. Be yourself (only more conversational, based on what you said above)