See the Java Console on Mac OS X
By vbkraemer on Dec 22, 2005
I soon discovered that other folks had hit the same issue, and some hints at a solution. Note: the response from the developer, tjquinn, is one of the most complete responses I have ever read in a bug report. All I needed to do was translate his response into Apple.
After some poking around, I found the 'Java Preferences' utility in /Applications/Utilities/Java/J2SE 5.0. This utility let you set the proper flags to open the Java Console.
The Java Preferences interface: Java Console radio button visible
Once I set it up, I was able to see the output of batch oriented application clients that get launched by Java Web Start.