According to the documenation, you can enable java console tracing by (jdk6)

1. entering a number at java console
or
2. set a couple plugin runtime properties via java control panel

but the #2 has no effect other than updating deployment.properties with the following line:

deployment.javaws.jre.0.args=-Djavaplugin.trace\=true -Djava=basic|net|cache|security|ext|liveconnect|temp.trace\=true


Excerpt from http://download.oracle.com/javase/6/docs/technotes/guides/deployment/deployment-guide/tracing_logging.html

"Tracing can be turned on by enabling the property javaplugin.trace. However, it turns on all tracing facilities inside Java Plug-in. To enable more fine-grained tracing, javaplugin.trace.option may be used. You can set trace-level options (0-5) in the Java Console, shown in the previous chapter, with the following meanings:

    * 0 — off
    * 1 — basic
    * 2 — network, cache, and basic
    * 3 — security, network and basic
    * 4 — extension, security, network and basic
    * 5 — LiveConnect, extension, security, network, temp, and basic

This enables tracing on the fly.

Another way to set fine-grained tracing is through the Java Control Panel. For instance, to enable tracing for everthing (option 5 above), enter the following in the "Java Run Time Parameters" textfield:

    -Djavaplugin.trace=true
    -Djavaplugin.trace.option=basic|net|cache|security|ext|liveconnect|temp

Tracing set through the Control Panel will take effect when the Plug-in is launched, but changes made through the Control Panel while a Plug-in is running will have no effect until restart."


There was a forum discussion about this issue:
http://forums.oracle.com/forums/thread.jspa?messageID=5211649