So if I post a meme to c/tenforward, people on mastodon and pixelfed can see it if they subscribe to @tenforward, is that correct?
Yep! I'm not sure how it works for pixelfed but I think I remember something about text posts so I would assume it works the same there.
Afaik the way to tell if a user is on Mastodon vs Lemmy/mbin/etc is if the instance part of their username is a path instead of a bare domain, it's something like user@instance.com/users
or something that includes "user" in the path, I can't remember exactly and I can't find an example but if you look at usernames enough you'll find one.
Edit: Actually maybe that changed because I found a comment from Mastodon and it looks normal just with a domain that you can see is a mastodon instance when you visit it, so idk ¯\_(ツ)_/¯
Edit 2: I double checked in the sync app and it looks like users from mastodon show up as someone@instance.com/users/someone
so it looks like it's a UI specific thing
Sure here's one I found of a Mastodon user making a post in a Lemmy community, they also comment so you can see how that looks as well:
https://feddit.org/post/12492506
Conveniently the post is about how interoperability between Lemmy / mastodon works haha