FreeBSDwiki:Filesystems, finding the type of
From Ubuntuwiki.net
Got a disk with an unknown filesystem on it? The file command ought to help you figure it out.
server# su server# file -s /dev/ad4s1a /dev/ad4s1a: x86 boot sector, code offset 0x3c, reserved sectors 0, Media descriptor 0x0, reserved 0x16, dos < 4.0 BootSector (0x1f), BSD disklabel
Note the su - you'll need it, unless you're already root.