The 'Connection information' panel in the NoMachine client menu (ctrl+alt+0 or click on the page peel on top right to open it) displays the RT (Real Time) port in use for UDP communication, e.g.:

Service type: NX on port 4000 with RT port 4016

Port displayed here, however, could be not the port effectively in use. The client displays always the port that was chosen on server side, even when the port effectively used is that one chosen on client side.