1) You won’t be able to see individual mailboxes in NT backup
2)Incremental backups are only logs.
If you want to confirm it is backed up, Do a verify when you backup and check the ntbackup logs and if you want to do it properly, restore them to a test server! If you have Exchange 2003, you can restore Mailboxes to Recovery storage groups on the live server without touching the live mailboxes!
Hope that helps…