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If you need to check what version of SharePoint server your Office 365 tenant is running especially during the Office 365 and SharePoint Online service upgrade (aside from checking through the Admin Portal via https://portal.microsoftonline.com) then you can add the following /_vti_pvt/service.cnf to the end of your SharePoint site – as shown below.

https://jcallaghan.sharepoint.com/_vti_pvt/service.cnf

The page will output two lines of text from which we can determine the version of the SharePoint servers. If the second row starts with 14 then you are running SharePoint 2010, if it starts with 15 then you are running SharePoint 2013.

SharePoint Online on SharePoint 2010 servers:

vti_encoding:SR|utf8-nl
vti_extenderversion:SR|14.0.0.6120

SharePoint Online on SharePoint 2013 servers:

vti_encoding:SR|utf8-nl
vti_extenderversion:SR|15.0.0.4454

After the service upgrade, you may be running SharePoint 2010 on SharePoint 2013 servers (technically known as 14 mode) until you upgrade your site collections to SharePoint 2013 (15 mode).