How do I tell if my email and calendaring data is on the Exchange On Prem Server or using Office 365?

The easiest way is to test which web email system you can access.

• Office 365 uses: Outlook on the Web (be sure to use your full username@mit.edu email address)
• Exchange On Prem uses: Outlook Web Access

See also:

Office 365 - Email and Calendaring Landing Page