Check version and subscription with NoMachine 7 and later
Launch NoMachine from Programs or Applications.

Click on Settings -> Server -> Updates

You can also access this information via the Connection Monitor running in the system tray: click on the !M icon and choose Show server status -> Updates

If you have the Enterprise Client installed, click on Settings -> Updates
 

Check version and subscription with NoMachine 6 and earlier
Launch NoMachine from Programs or Applications.

Click on  Settings > Server preferences > Updates.

Or click on the !M icon  in the system tray and choose Show the service status ->  Server preferences -> Updates.

Check version and subscription manually
As an alternative, you may check the server version and subscription by command line. 

On Linux and macOS open a terminal and execute respectively:

sudo /etc/NX/nxserver --version
sudo /etc/NX/nxnode --version

sudo /etc/NX/nxserver --subscription
sudo /etc/NX/nxnode --subscription

On Windows, open a CMD console as administrator and move to the 'bin' directory under the NoMachine installation:

cd C:\Program files (x86)\NoMachine\bin\

and execute respectively:

nxserver --version
nxnode --version

nxserver --subscription
nxnode --subscription

Output is for example:

sudo /etc/NX/nxserver --version
NoMachine Enterprise Desktop Evaluation - Version 7.0.206

sudo /etc/NX/nxserver --subscription
NX> 622 You appear to run an evaluation version due to expire
NX> 622 on Fri Dec 25 08:51:37 CEST 2020. Please visit the
NX> 622 NoMachine Web site at http://www.nomachine.com/
NX> 622 to acquire a valid subscription.