If you have inherited a XenDesktop environment or you are just doing an audit and need to know which version of XenDesktop version you are running you can use PowerShell to retrieve that information for you.  The command to issue is:

Get-BrokerController

This will give you similar output to below where you can view the version by looking at the ControllerVersion. In this case is shows I am running 5.6.0.33