I have a new coworker who happens to be a person of cisgendered experience.

They seem nice and everything, and I want to be respectful, but I'm confused. Like, how do you know what pronouns to use?? Normally, I would just ask, but I've heard that that's offensive in the cis community??? One cisperson I talked to said that their pronouns should be obvious to me just by looking at them, which is frankly ridiculous. I hate to say this, but it seems impossible NOT to offend them. I honestly just don't get it. What should I do?