How to find Base/original disk of VM which is running from snapshot?

in our backup solution..

1. client VM's snapshot will be taken and VM's base disk will be attached to backup proxy nodes.

After backup completes VM's base disk will be detached from backup proxy nodes..but sometime base disk remains in backup proxy nodes.

so i am looking for script where i want to find base/original disks of $vm and compare it with $backupnode and remove that disk from virtual machine $backupnode.

But problem here is $vm has snapshot and i am not able to get the base/original hard disk name... could someone help me on this?

$vm="abc"

$backupnode="xyz"

regards,

Ranjit