You can use the CLI to determine whether there are deleted messages for a particular mailbox. From the CLI, use these two commands:
show cuc dbview contents unitymbxdb1 w_mailbox
show cuc dbview contents unitymbxdb1 w_message
This will generate two files, one containing the mailboxes and one containing the messages. Find the user of interest in the file containing vw_mailbox by searching on the user's alias. There will be one line per mailbox. The top line contains the column names. In the line for your user, get the value of the MailboxObjectId column. In the file containing vw_message, there will be one line per message. Search for your user's MailboxObjectId. There is a column Deleted that will show a value or 1 if the message is soft-deleted.
To actually remove the deleted messages from the deleted items folder, the user will need to erase the deleted files through the TUI, IMAP client, or CPCA inbox.