querying free space on a dvd-rw disk

[adler-wRvlPVLobi1/31tCrMuHxg at public.gmane.org (Stephen Adler)]

Hi Blu'ers...

I'm writing some scripts to do backups of data for my job. Does anyone 
know how to query a DVD-RW disk to find out how much free space is on 
it? I guess one could mount it, get the used space and subtract that 
from the official 4.7Gigs of DVD capacity, but it would be nice to be 
able to do some kind of direct query.

Thanks in advance.

Cheers. Steve.