This isn't quite as simple an answer as you want, because the unit that's assigned to a storage policy isn't a client, but rather a subclient, but there is a View in the SQL DB that you can use to get the answer you want. You'll want a CommServDBQuery activity with something like:
where storagepolicy = '***Test'
If there are no subclients associated with a given Storage Policy, you'll get a count of 0 there.
Obviously, it's a Good Idea to play around with that query a bit in SQL Server Management Studio before letting that loose in a workflow that kicks off Storage Policy deletion. (For example, you should probably do a regular old select * to see what all's in the table.)