the file in question is /proc/filesystems (just cat it).
My generic debian stable box doesn't show UFS.
From a little work on google:
1) there is a kernel module called 'ufs' which may allow read-only access.
2) -o ufstype=openstep must be used on the mount command, as there are several ufs variants.
3) It appears UFS support isn't well documented at all, and may or may not build.
I know this though, its trivial to do in FreeBSD.