Hi Rich,
     We have both CLI and API to do it.
Below is the cli.
[root@lnx ~]# dfpm dataset snapshot reclaimable help
NAME
     reclaimable -- compute space reclaimable from deleting snapshots in a volume
SYNOPSIS
     dfpm dataset snapshot reclaimable <volume-name-or-id> <snapshot-name-or-id>
         [ <snapshot-name-or-id> ... ]
DESCRIPTION
     Compute space that can be reclaimed if the specified
     set of snapshots are deleted from the given volume.
[root@lnx ~]#  dfpm dataset snapshot reclaimable 1615 21050 hourly.2
Approximately 457868KB will be reclaimed by deleting these Snapshot copies.
[root@lnx~]#
PS: Though its a dfpm cli its not necessary that the volume has to be a member of a dataset.
The corresponding is the API.
<snapshot-get-reclaimable-info> 
<snapshots>  
<snapshot>   
<snapshot-name>21050</snapshot-name>  
</snapshot>  
<snapshot>   
<snapshot-name>hourly.2</snapshot-name>  
</snapshot> 
</snapshots> 
<volume-name-or-id>1615</volume-name-or-id>
</snapshot-get-reclaimable-info>
Regards
adai