The generic file system commands determine the file system type by following this sequence:
From the -F option, if supplied.
By matching a special device with an entry in the /etc/vfstab file (if the special device is supplied). For example, fsck first looks for a match against the fsck device field. If no match is found, the command then checks the special device field.
By using the default specified in the /etc/default/fs file for local file systems and in the /etc/dfs/fstypes file for remote file systems.