It's designed to be seamless so that you don't notice if posts are from other server types, but there is a way of checking!
If you go to a post's original page, it will open it on the account's own server's website. The server's type will be obvious from the website's layout, logos etc.
Here's how to open original pages:
https://fedi.tips/what-are-original-pages-in-mastodon/
You can try this on the accounts I linked to in the first post of this thread, they will all open on non-Mastodon server websites.