In my experience, this tends to be one of those "it's not what you say, it's how you say it" sort of situations.

Obviously if someone is wearing a button or something that says "they/them" or whatever, then go with that. In most other cases just kind of go with your gut, be respectful, accept that you might be wrong and if they correct you, apologize and make an effort to abide by their preference. They live in the world, they get it, it's new to a lot of people, it's not always intuitive, and it's almost definitely not a new experience for them. If you're respectful and make an effort, for a lot of people that's like 90% of what they want from you, and they'll roll with your mistakes.

And if you want to be safe, generally speaking, they/them works for pretty much everyone, and although there's some weird ambiguity in some cases of if it's singular/plural, it's grammatically correct. I personally make an effort to use it as much as possible whether I'm talking about a trans/nonbinary person or a cis person, and honestly it hasn't gotten anyone's feathers ruffled, most people don't even tend to notice. I may need to think my sentences through a little more than I would have otherwise, but thinking before you speak is probably a habit a lot of people could stand to develop anyway.