Bring up or wait until it is brought up something like relationships or dating. You might be able to hint that you are single, like joke about single life being annoying but nice, or some mixed statement that says you are comfortable where you are but you also like to date. By saying you are comfortable being single you are saying you are not desperate but as long as you also mentioning you wouldn't mind not being single you open up the door. Maybe make some stereotype tease about females but back it up that you still like them.
Bringing up the topic and your own feelings about it might get her to say something like "yeah, my boyfriend...blah blah blah" or, "yeah, my old boyfriend was...blah blah blah". once the topic is up, even if she doesn't explicitly say shes not seeing someone, its no longer tabboo or presumptuous to ask her in the context of the conversation if she has a boyfriend. like if she mentions her old boyfriend but doesn't actually say she is still single. And if she mentions an old boyfriend, her mind is still on that rather than on a current one, so its a good bet she is single, since a taken girl wants to talk about her boyfriend, not some old love. If you can do it in a teasing way that would help as well.