These are the notes for GoZync 3. Docs for the latest version of GoZync--GoZync 4--can be found here. GoZync 4 is a free upgrade and is highly recommended (hint: it's faster).
How can I test to see if a user has modified by un-synced records on their device?
If a user's device is offline, you can't test from the host or server, but you can test inside the scripts you put in their mobile file. In fact, it's pretty common to prevent users from pulling down new records if they have un-synced changes for that table.
When you integrated GoZync into your file you added a couple field to your primary table occurrence. One of these is "gz_Send"; that field will have a 1 in it if you've modified by not synced the record.