Some background first: Exchange 2003 stores the bulk of its configuration information in the Configuration partition of the Active Directory. As such, it can't be installed w/o a domain. Further, Exchange only allows for a single "Organization" per Active Directory forest.
If you just want to do test restores you have a couple of options:
Bring up a secondary Exchange Server computer and restore databases from your production server into a Recovery Storage Group on the secondary Exchange Server computer. This would test the ability to restore the backups to retrieve individual mailboxes. It doesn't test complete DR capabiltiy of the entire domain / Exchange infrastructure (since it assumes that the domain still exists), but a database that will restore into an RSG will restore into a "normal" storage group.
Put a DC from your current Active Directory and an Exchange Server computer into a "sandbox" network and restore onto them. You'll have to work out network connectivity because running them in the same subnet as existing DC's potentially will result in NetBIOS name resolution between them and inter-communication. This would be more of a disaster recovery test.
If you wanted to get really in-depth and test a "all the servers were destroyed and everything needs to be rebuilt" scenario, do the following:
In a sandbox network, perform a DR of a domain controller into a (virtual or physical) machine from your Netbackup backups. Remove replication connections and seize FSMO roles as necessary to end up with a machine that's a single domain controller holding all FSMO roles.
Perform a DR installation of Exchange (setup /disasterrecover) onto another machine in the sandbox. Name that machine the same name as the existing Exchange Server computer. Partition the disks the same, if possible, so that the databases lay in the same place as the production box.
Restore the Exchange databases from Netbackup onto the sandbox Exchange Server computer after marking all the databases on that server as being allowed to be overwritten by a restore (and don't create any RSG's).
If you can pull off that "oh, god, everything has been destroyed" backup / restore scenario then you've got good backups.