In general, it is not your concern whether a girl has a bf or not, it is hers. As men, it is our job to find out whether a woman is sexually interested or not- regardless of whether she is "in a relationship" or not. It's the woman's relationship to defend, not yours- it's super easy to reject you. Your question is, therefore, an invalid one. You want to find out whether she likes you? Invite her out, and make a move on her (ie try and kiss her).
However, it's been said many times on here, and for good reason: don't hit on girls at work! There are plenty of other available women out there, so why risk getting involved with someone who could wreck your career if things go wrong? You only have one job and there are billions of women out there... think over that carefully. Use your hobbies and your social circle to meet girls.