you could use
zmmailbox with
getAllFolders on your accounts and see how many items are in the appointment-type folders, any non-zero value should indicate the calendar is being used
for example
Code:
[zimbra@mail ~]$ zmmailbox -z -m user@domain.tld getAllFolders
Id View Unread Msg Count Path
---------- ---- ---------- ---------- ----------
1 unkn 0 0 /
16 docu 0 0 /Briefcase
259 docu 0 1 /Briefcase/blau
10 appo 0 7 /Calendar
14 mess 0 0 /Chats
7 cont 0 1 /Contacts
6 mess 0 0 /Drafts
13 cont 0 6 /Emailed Contacts
2 mess 2 5 /Inbox
4 mess 0 0 /Junk
5 mess 0 1 /Sent
15 task 0 0 /Tasks
3 unkn 0 0 /Trash In this case, the user has 7 items in their
/Calendar folder.
This is the first idea that comes to mind ...there might be a more efficient way of doing it.