You can typically tell this from the URL you visit to read your email. If you are using Office 365 it will be https://outlook.office.com/mail/. If you are using Exchange it will likely have your company domain in it, such as mail.mycompany.com.

1. Create a new message or open an existing message and click on the More actions icon.
2. Click on Get Add-ins.
3. Select My add-ins → Add a custom add-in → Add from URL and use the following manifest URL: https://static.netcraft.com/reporter-button/manifest.xml

The add-in currently only supports the mobile Outlook Apps when you are using Office 365. In addition add-ins cannot be used on the mobile website of the Outlook Web App (OWA).