I'm using a command that reverts a diff chunk in a buffer (linked question), however I don't know if the buffer is newly opened - or if an existing buffer is used.
While I could make a list of all buffers, then check if that buffer is in the original list, is there a better way to know if the buffer from a function is newly created or not?
"Newly opened" can only mean relative to a state when the buffer didn't exist. Every time prior to when it came into being it didn't exist.
So yes, you need to take a snapshot of the list of existing buffers at some point prior to creating the new buffer, in order to later check whether the buffer is "new" - compared to that prior snapshot moment.
Save the value of buffer-list in a variable (e.g. let-bound), and then, later, check whether the buffer in question is in that previous snapshot. If it is, then it's not new; if it isn't, then it's new.
If you're only interested in certain kinds of buffers then you can filter buffer-list, to make your lookup quicker.