How can I test to see if a user has modified but un-synced records on their device?
Yes. 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 fields to your tables. One of these is "gz_Send"; that field will have a 1 in it if you've modified but not synced the record.
Note that the script "Zync - Is a new version permitted" you added to your file does exactly this, though you'll likely want to extend it to check all the tables in your file.