Previously I posted an example on Mining the Client information on Sent Messages in EWS to report on what client is being used in this post . For Calendar appointments there is a simular but different property that gets set when you either create or update a Appointment eg
(This property is set by the Exchange Store based on what client is being used)
I've put together a script that aggregates this property and produces a summary report like
Its also produces an export report of all the appointments that looks like the following so you can see at the appointment level what client is being used.
I've put this script up on GitHub here https://github.com/gscales/Powershell-Scripts/blob/master/ShowCalendarClients.ps1