To find Plesk version, login to server as user root SSH and run
plesk version
Example
root@root861:~# plesk version
Product version: Plesk Onyx 17.9.8
Build date: 2018/11/16 16:00
OS version: Debian 9.6
Revision: 6af2ef23569cac03e01272676158a016ed0eea1a
Architecture: 64-bit
Wrapper version: 1.2
root@root861:~#
