Your pronouns are whatever you would like to be referred to. Generally someone would either correct you or you would hear the right ones during conversation to learn someone else’s. If they outwardly present as a specific gender then I would normally assume (or default to they/them) and just apologize and correct if someone corrects me. Most normal people will take such an interaction in stride without further thought.
In terms of online, people often add it to their profile so you know, or to indicate to others that you respect their choice of pronouns.