Revision History lets you click through all the changes in a document, along with a timestamp and the person that made the change. You can restore older versions of a document, or just see what changes or progress has been made (and if any large chunks of text were posted or if any other user made contributions). This can corroborate (or disprove) a student that claims to have been working really hard and but not making progress by demonstrating when they've accessed or modified the document.
It's easy to find.
The revision history is grouped into time periods but can be expanded to see individual changes.
Eric Curts, a Google Education Trainer, has a more detailed post on avoiding and addressing cheating using Google Apps here: http://www.controlaltachieve.com/2016/01/student-cheating-google-apps.html