These steps are for TFS 2012 and above:
TFS Administration Console:
The TFS Administration Console shows the installed version when the Application Tier icon is selected. However, this requires logging onto one of the application tiers and having the rights to open the console.
Web Browser Method:
This is by far the fastest method and available to all users.
Using a web browser, log into the TFS instance. For example, http://10.0.0.1:8080/tfs
Select the Question mark on the upper right of the browser in the blue bar
Select About in the drop down and it will look like this:
This method is handy if you wish to view the version per team project collection to ensure all were updated to the correct version:
SELECT * FROM fn_listextendedproperty(default, default, default, default, default, default, default) WHERE Name = 'TFS_PRODUCT_VERSION'