What file or directory is unique to Plesk and unlikely to change in the future?

I am writing a program that needs to be able to detect the presence of various web hosting control panels. I don't use Plesk, or have access to a server running Plesk, so I am unable to ascertain what file or directory I could test for to reliably conclude that the system is running Plesk.

What file or directory indicates "Yes, this has Plesk installed" that is unlikely to ever change in the future, but would be present on all versions of Plesk out in the wild?

Thanks in advance :)

From serverfault Tim Post

• If Plesk is installed there should be a version file at this location on Linux systems:

  /usr/local/psa/
  

  Or you can check for the directory itself. If needed you can use the version file to determine the installed Plesk version. According to the Parallels knowledgebase, Plesk won't even start when this file doesn't exists: http://kb.parallels.com/en/3968.

  I'm not the right person to tell you how to detect Plesk on Windows based systems ;).

  Tim Post : Thanks! I appreciate it.
  From PowerSp00n