how to get 'real' freespace after rsync+hardlinks?

[snowcrash+rsync]

ok, so this is not entirely an rsync question.  but, it seems the
'crowd' most familiar w/ hard-linking are the rsync crowd. so ...

i'm doing rsync + incremental backups to a local drive.

the incremental steps, currently, use the

  rsync REMOTE_DIR OLD_DIR
  ... time passes ...
  cp -al OLD_DIR NEW_DIR
  rsync REMOTE_DIR NEW_DIR

approach.

all works great.

i'd *like* to be able to check my LOCAL (destination) drive for
sufficient space b4 doing a backup.

question:  how does one check "real" available freespace?  "real", as
in not double-counted hardlinked/shared files.

fwiw, my LOCAL drive is on an osx/hfs+ box (i actually think this may
make a difference, esp wrt linux ...).  'native' osx utils (finder,
diskutil, etc) report the apparent usage -- not the acutal,
hardlinked/shared usage.  as a result, i can't readily check for
space.

what do folks here do?

thanks.