Scenario: Can we identify users that are using Cached Mode or Online Mode from their Outlook client when connecting to their Exchange Online Mailbox via Microsoft Online tools or reporting?
Answer: No – At least not as of today, 11/17/2020.
Microsoft Online currently does not provide this level of detail of connection data for Outlook MAPI connections. In Exchange On-Premises, you had this information stored in RPC logs that could be parsed through. In Exchange Online, we do not have the access.
In order to determine if Cached Mode is enabled for the Outlook client connections, you would need to perform logic on the client initiating the connection.
Ex-Shell 