Noted, you can remote to check by the Command line.
1. Run CMD to open the command Prompt window
2. Run netstat -no
3. Search 3389, if there is a line as below, the customer must use the remote desktop way.

By the way, if they installed ZWCAD on a server, and use ZWCAD stand-alone license by several users at the same time, it will have the same result. I don't know any other way to connect the server, the remote desktop is the way I know, maybe there is some other way, like terminal client, or thin client.

You just need know that, ZWCAD can only be used by one user for stand-alone license.