[root@amex freebsd-testing]# losetup /dev/loop3 /freebsd-testing/freebsd-8.0p2-i386-xen-domu.img
[root@amex freebsd-testing]# efsck -f /dev/loop3-bash: efsck: command not found
[root@amex freebsd-testing]# e2fsck -f /dev/loop3
e2fsck 1.39 (29-May-2006)
Couldn't find ext2 superblock, trying backup blocks...
e2fsck: Bad magic number in super-block while trying to open /dev/loop3

The superblock could not be read or does not describe a correct ext2
filesystem.  If the device is valid and it really contains an ext2
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193 <device>

[root@amex freebsd-testing]#