Don't assume "cis until proven otherwise".
There are plenty of trans people who aren’t out, both due to personal reasons and reasons of safety in a society that is still incredibly hostile to trans people. This goes for trans people in all situations, whether or not a person wants to, has chosen to, or is able to transition in any way.
Regardless of how someone appears to you, you don’t know that they’re cisgender unless they directly tell you — so it is important to always use language that wouldn’t alienate a trans person in the room, even if you don’t think anyone in the room is trans.
Being a good ally isn’t just about amending the way you refer to people when they directly ask you to — it means actively deconstructing the assumption that cis people are the default until proven otherwise and trans people are the exception (as well as the assumption that everyone is male or female). In practice, this means making no assumptions about a person’s gender, pronouns, or biology until they tell you, and behaving towards everyone you meet in a way that reflects that. In particular, it means eradicating binary language and the gendering of people without their consent, especially in institutions such as universities and workplaces where people often don’t want to or can’t come out as trans.
Content Warning: this page includes discussions of cisnormativity, gendering without consent.