"Knowing" at first sight isn't very important. Why does it matter? Not everyone has to (or wants to) broadcast their identity with shirts, slogans, grand proclamations, or some kind of general attitude or bearing. Just treat people like people.

If you address someone by an incorrect pronoun the first thing you should do is simply apologize -- you don't have to grovel at their feet and declare all the ways in which you're wrong -- and ask what pronouns they prefer.

"Oh, I'm sorry. What do you prefer?"

It's okay if you don't know. It's okay if you make a mistake. If they don't accept your (assumed genuine) apology and attempt to understand/make things better, then that's on them, not you.

Don't take one person's reaction and apply it to everyone who calls themselves trans or non-binary or genderqueer. I've had liberals get up in my grill because I've used the wrong pronouns before, taking it as the greatest offense they've ever been subjected to, while radical soon-to-be friends understand that mistakes happen and, as long as you aren't a fucking asshole about it, it's okay to make a mistake.