How can I determine if a question is off-topic or if through editing by a third-party was made off-topic? Unless, there is some way to see what the edits were, this seems problematic.
When you click the "edited ..." link...
you'll get to the "Revisions" page, which shows every revision that was created by users editing the post, automated Community actions or other events like questions being closed or locked.
The page is also available by appending
There is also a more specific timeline, which allows you to inspect which actions happened when. It contains much more information than just the post revisions.
To access a timeline, you have to manually enter the URL:
See this question for more information: Feedback Request: New Timeline Question View
You can view the history of the revisions to a post.
If a post has been edited, there will be a footnote on the post saying when it was edited and who edited that you can click on. Alternatively, you can change the URL of a post to view the revision history.
For example, this is the revision history for this question: