Add System.out.println() to inspect the state of your application. Output goes to the JavaScript console when executed on the browser. When executes on the server, it goes to the Tomcat console.

In addition, you can use the Management Console inspect the state of your application. You might want to try the embedded Grooxy shell.

You can debug the server part of your application as follows:

1. define an environment variable MAVEN_OPTS=-Xdebug -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n. (Set suspend to y if you want to debug the compile.)
2. start your application: mvn qx:run
3. start your debugger for a remote application on port 8000