In the past I had an attempt to do this by creating an SQL Query, but it is a bit more tricky than you might suspect. Schedule Policies can be applied to Clients, iDataAgent, Instance, BackupSet, Subclient, Client Computer Group, and iirc Subclient Policies. Then, it is possible a client is Scheduled but not via a Schedule Policy.
There is a general Command Centre Report in the Commvault Store for schedules, but found that the resultant webpage ran very slow if you increased the number rows. In summary, I'd recommend you raise a Customer Modification Request over a ticket.