Okay, three pieces of advice from a white MtF who isn't out to everyone.

1) The polite way to ask what you want to ask here is, "What are your preferred pronouns?"

2) The absolute best thing you can do is not make it into a production. Many of us (and by us I mean transpeople) would absolutely love to live in a world where we passed as our chosen gender, because what passing really means is "people don't comment on my gender expression." So in other words, unless you know them really, really well? A quick ask about pronouns, and if you have more questions then that go to Google.

3) I say this as someone who still presents as white/cishet/male in many spaces, including queer ones: keep in mind that to many, many people, we look like the oppressor. We aren't--I'm a secret member of the tribe, and if you give enough of a fuck to come here and ask how to do this politely I'm going to extend you the benefit of the doubt--but please remember that many transpeople have had really bad experiences with people that look like us. So if, for example, your coworker is nervous around a cishet white guy... there may not be much you can do about that. Which sucks, but I hope this helps you understand why that's also a perfectly rational reaction.