How to Determine the Version of the Citrix License Server

Look at the following registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\LicenseServer\Install (for 32 bit operating systems)
HKLM\Software\Wow6432Node\Citrix\LicenseServer\Install (for 64 bit operating systems)

<version> 11.6.1 build 10007 is for XenApp Server 6.0
Note: Version 11.6.1 has multiple releases and is distinguished by build numbers (previous values are 9018 and 9020).
<version> 11.5 is for XenApp Server 5.0
<version> 11.5 is for Presentation Server 4.5
<version> 2.0 is for Presentation Server 4.0
<version> 1.0 is for MetaFrame Presentation Server 3.0