Google Documents changes are typically batched into groups. I'm not 100% sure what the criteria is, but small changes within the span of a few minutes should all be grouped together.
If you're looking for a way to see both if and how you've changed the document's history, then go to
See revision history
You can also get there by using Ctrl + Alt + G
In this history just click on the entry that you want to view and click
Restore this version
To see even more detailed revisions (not grouped by time) - then open the Revision history and click on
Show more detailed revisions at the bottom of the menu.
*Edit: Based on the question in the title you might also be looking for a way to disable autosaving. There is no way to disable auto-saving, but based on another Webapps answer there is a clever workaround:
After you login to Google Docs and are looking at your list of
documents, hover your mouse over the document in question. You should
see an 'Actions' dropdown appear to the right. If you click on that
you should be given the option to 'download' the file.
Now you can work on your document locally on your office computer and
when you are done for the day just upload it back to Google Docs. :)