I'd probably do something like this:

1. check for /dev/dvd, /dev/cdrom, /dev/cdrw, /dev/dvdrw
2. mount the device to some directory and see what happens. on my system mounting without a disc reports "mount: No medium found". if it succeeds you have a formatted disk, another error probably means the disc isn't formatted or formatted using some unusable filesystem.
3. see 2.
4. see 2.
5. skip this, growisofs will check if there's enough space.
6. check the output/return code of growisofs.

update: also please don't repeat post. it takes some time for posts to be approved and they won't be visible in the right section until they are (and repeat posts will be removed, but this takes up time and attention from the monks that might be better spend somewhere else). New posts are immediately visible in Newest Nodes.