SharePoint MVP Serge Luca just posted an article with the title “Can you check the version of a SharePoint Online ? (or how can you know that your SharePoint Online moved to 2013…)“. In it, he shows how you can find out which server version your SharePoint Online tenant is running on, for example if you want to know whether SPO has been patched with the latest CU, or currently to see if you’re still running v14 (based on SharePoint 2010) or v15 (the brand new SharePoint 2013).
There are 2 other ways to find out the version number:
The easiest way is to simply open https://yourtenant.sharepoint.com/_vti_pvt/service.cnf (or http:// if you’re using the P1 plan) , which will give you a result similar to the following:
As you can see, my own tenant is currently on 188.8.131.5233
Use Fiddler to see response header
The other way is to install and use Fiddler. If you run it (don’t forget to activate Decrypt HTTPS traffic in the Options) and open your SharePoint Online site, any response header will include the current version on the Headers tab, in the Miscellaneous section, under MicrosoftSharePointTeamServices: