Revision History lives in nearly all of the Google Apps, and, in my opinion, it is one of their very best features! To find it, just click on the File menu in Docs, then on Revision History (or if you have a whole bunch of very flexible fingers, you can use the keyboard shortcut Command+Option+Shift+G).
Once you’re in there, be sure to click “Show more detailed revisions” to see all you need to see. Clicking on any given time will show you how the document looked at that very moment. Depending who is editing the document, any changes they have made will correspond to the color beside their name in the Revision History list.
Here are some super awesome reasons why you might want to dig into Revision History: