The easiest way is to look under the following registry key:
There you have two values that might provide what you need:
"DisplayName"="Thinstuff XP/VS Terminal Server 1.0.390"
Another more complicated option would be to look at the version resource of the ThinrdpAdmin.exe file.
Please see MSDN info about the Resource Version Information.
In order to retrieve the full path of the thinrdpadmin.exe file you first have to query the registry for the Thinstuff installation folder and then append the filename.
Tags: code, detect xp/vs version, Version Information, version resource, XP/VS Server
The installation folder is saved under [HKEY_LOCAL_MACHINE\SOFTWARE\Thinstuff\XPVS]:
"InstallDir"="C:\\Program Files\\Thinstuff\\XPVS Server\\"