When you look at a document you'll see that pages have a number of different statuses:
You'll see this same status on the overview tab for the page.
What sets that status?
A page with no work is "not started"
If you transcribe part of a page and click "save", then the page is "incomplete"
When the transcription is finished and you click "done" then the page is "complete"
The "Needs Review" state depends on how the project is configured. Some projects are configured so all completed pages need review. Other projects ask that you check the "needs review" box after it's done only if you feel the page needs another set of eyes.
So how do you find pages that need work? You can scroll through the pages on the document's page or you can click the "contents" tab to find pages that are not started, incomplete or need review. The easiest way, however, is to use the "Pages needing transcription" or "Pages needing review" buttons on the document's page.