We can't always know for sure and people frequently guess wrong. Because of that, it's important not to lead by attacking the other person, especially when they account is new, because newcomers usually don't know anything about the conventions of this site. Under these circumstances, it's mean and destructive to blast them with rudeness. Rather, we should be welcoming them and gently educating them. If they're a good new user, they'll respond well to that (you can see an example of this here: https://news.ycombinator.com/item?id=46682308). If they aren't a good new user, that will eventually become clear and the immune system (flagging, moderation, community members emailing [email protected], etc.) will eventually take care of it.