How did you get that 'good sense' with forums back in the day? I did that by reading into the community. Just look around on the server, see the posts on there, and see if there's a connection. You can do that with Mastodon servers too. And it's not that high stakes. You are not stuck on that server, if you find out that it's not quite the right fit, you can move to another server. In fact, if I were to look at my following list, I think I see more people from outside the mastodon server I joined than people from the same server.