How you can find out which server version your Sharepoint online tenant is running on?for example if you want to know wheteher SPO has been patched with latest CU or currently to see if it is still running v14(based on SharePoint 2010) or v15(based on SharePoint 2013).

There are two methods to find out the version of SharePoint online:

Open /_vti_pvt/service.cnf

The easiest way is to simply open https://yourtenant.sharepoint.com/_vti_pvt/service.cnf (or http:// if you are using the P1 plan), which will give you a result similar to the following:

vti_encoding:SR|utf8-nl
vti_extenderversion:SR|16.0.0.7925

As you can see, my own tenant is currently on 16.0.0.7925

Use Fiddler to see response header

The other way is to install and use fiddler.Run fiddler and open SharePoint online Site. Don’t forget to activate decrypt HTTPS traffic in the options. Any response header will include the current version on the headers tab, in the Miscellaneous section, under MicrosoftSharePointTeamServices:

SPOVersion