Most people aren’t going to ask about your pronouns if you present in a traditionally gendered way. If someone tells you their pronouns I think it’s polite to tell them yours too even if you think it’s obvious. It sounds like the way you’ve described yourself that your pronouns are he/him. If you find yourself around a group of queer/trans folks it would probably be contextually appropriate for you to introduce yourself as “My name is and I use he/him.” If you want to go above and beyond and do that all the time nobody will fault you that would benefit or appreciate that information. Some people may be confused or make an assumption but if you want to be an ally that can include taking the brunt of some of those conversations and teaching people why you’re doing it. Makes it more normalized.
You are pretty much always welcome to ask someone’s pronouns if you are unsure. Most trans/non binary folks will appreciate you asking because unfortunately there are a lot of cis hetero men that wouldn’t extend that courtesy or demonstrate that they’re accepting and open like that. As long as you’re respectful just ask “Hey, name, what are your pronouns? Mine are he/him.” and that will be more than sufficient. If you mess up after that do not apologize. Instead, thank them, restate your sentence using the correct pronouns, and move on. Don’t make it a huge ordeal and fall over yourself apologizing. Saying you’re sorry in that situation puts the misgendered person in the position of having to say “it’s okay” when it’s not. Thanking them demonstrates you’re aware that you made a mistake and shows that you are appreciative and trying to learn and do better.
For example: You: “He loves to skateboard.” Other person “they” You: “Right! Thank you. They love to skateboard.” then continue the rest of what you were saying.
Online can be tricky. You only really need to gender someone if they gender themselves. Everyone to me online is they until I see evidence otherwise. It makes your life easier to just be as neutral as possible.
There are definitely people out there that use unconventional pronouns. I’ve only met a few that use something other than they/them but they’re out there. It’s not just a fad or for inclusivity for the sake of it, it’s just rare. In fact there are probably people you’ve met that use they/them but they may not have been comfortable telling you that. It’s understandable, but I think it says a lot about your character that you’re curious and want to do the right thing. Thanks for asking!