The fsck available in FreeBSD's base system doesn't work on Linux file formats such as ext2 and ext3. For that end one first needs to install sysutils/e2fsprogs.
Bellow I'll explain how to install sysutils/e2fsprogs and use the supplied fsck.ext2 on a ext2 partition.
First, install the port:
# cd /usr/ports/sysutils/e2fsprogs
# make install clean
Now the fsck.ext2, fsck.ext3 and fsck.ext4 commands are available. Assuming the /dev/ad4s1 as being the faulty partition, run:
# fsck.ext2 /dev/ad4s1
e2fsck 1.42 (29-Nov-2011)
/dev/ad4s1 was not cleanly unmounted, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/ad4s1: 37/10040 files (13.5% non-contiguous), 13062/40128 blocks
And that's it ;)