Hello! Resident Genderfluid person here. Usually you can just ask their name and that works to get them to give you the greeting they like. They look like a James, but give you the name Samantha? Probably safe to use she/her unless doing so has them ask you to use something else. IRL, at least.
Online? It’s usually in a bio or they will tell you if it is functionally relevant. The only people I (anecdotally) have seen devolve into scree when accidentally misgendered were people trying to start something or acting for the sake of poisoning the well.
As far as using non conventional pronouns irl, you probably haven’t heard it since it is genuinely dangerous to be outed in a lot of places. Look up gay/trans panic laws. It’s dangerous to be queer in America with Conservatives having so much sway right now.