I'm assuming you're on a pre-8.2.4 version. It'll be slightly different in 8.2.4, so I'll outline both:
pre-8.2.4:
-manual backups result in backup file in /alternative as 'databackup.tar.gz'
-check backup size, compare against backup size in /var/airwave-backup
-see backup log: /var/log/nightly_backup to verify backup completion, last line of a backup should be '(finished tar)'
post-8.2.4:
-all backups are visible from CLI in listing format when you view 'download file' option, daily backups are named 'nightly_data###.tar.gz', manual backups are named 'ampbackup_date.tar.gz'
-again, compare backup sizes
-if you download the backup package from system, you can open and view the nightly_backup file to check to see if complete