In the "November 2019" release command line options to get the client versions were added.
sudo /opt/skopos/skopos_client.sh --version
For Windows you can use the following command:
"c:\Program Files\Skopos Security Labs B.V\Skopos Agent\SkoposWindowsClient.exe" /version
In Windows you can also check the version by going to Settings -> Apps and Features -> Search for 'Skopos' (or manually find the Skopos installer) -> Select the Skopos installer to view the current version.