From IzPack 3.11 on, there's a built-in debugger for variables and conditions. The debugger is activated when the installer is called with -DTRACE=true as JVM option.
java -DTRACE=true -jar myInstaller.jar
The debugger shows up as a panel in the east of the InstallerFrame or if the guiModifier showDebugWindow is set to true, it will show up as seperate window.
<modifier value="true" key="showDebugWindow">