I found this in GWT sample Dynamic Table. I love this function b/c it prints out the object that will be sent in the RPC. Put this snippet in your server side of the rpc call, MyServerImpl.java class.

//stick this in your server side, MyServiceImpl.java class
/**
* Write the serialized response out to stdout. This is a very unusual thing
* to do, but it allows us to create a static file version of the response
* without deploying a servlet.
*/
protected void onAfterResponseSerialized(String serializedResponse) {
System.out.println(serializedResponse);
}