When you run the migration in the command line, you can see a broad view of how much progress has been made for each Class. Notice the last line is called the "second pass". I'll explain that more later.
Going to example.com/admin/migration_report/ will take you to this page, showing similar information on your migration progress.
The live update button on the bottom tells the page to refresh the report every five seconds.
You'll notice as you first start importing, that all of your media objects will jump to either "Failed" or "Waiting". Media objects in "Waiting" are waiting for the "second pass" (mentioned before), which will connect them with their masterfiles and derivatives. During the second pass these object will move to either "Failed" or "Completed".
You can drill down to a class type and see items and some basic migration info. This is a good place to check for migration errors to work on.
Clicking on the an item, brings up a detailed view of any datastreams that are attached to the item.
You can look at the datastreams (including any images) below the header under Fedora 3 or Fedora 4.
Some migrated items won't have fedora datastreams attached to them. There are some items that are migrated straight from the database. For now they have limited details pages. After you run the avalon:migrate:db rake task, the db items will also show up in your migration report.