The real answer is that it is extremely difficult to know. It's very easy to find the disks in use by LVM or VxVM. It's easy to find those used as swap or filesystems. It's easy to find those used as raw disks by databases. You can use ioscan -C disk -fn to identify all the disks visible to the system. The problem is that after identifying all the disks and accounting for the ones that are obviously in use, you may still have some disks left that APPEAR to be unused. They may very well be unused but they may not as well. It's possible (though not common) for an application to use a raw disk --- and unless you know the application there is no way to tell that this disk is in use.
The real answer to your question is to document, document, document.
If it ain't broke, I can fix that.