If I've never heard anyone else use a specific pronoun for someone new or I otherwise don't know, I try to use they/them. Otherwise I use what others do.

And if someone does let you know that a person/themselves prefer a specific one, always say thanks (you can't be sorry for something you didn't know!) and do your best to remember for next time.

I also try to use genderless terms like "folks" or just "everyone" instead of "guys"