If they don’t reveal them selves with the defaults read command, maybe you could use the strings command onto the executable, to see what kind of strings it may accept?
There is another thing you can do for discovering default values, and that is to open the package of an app, and look for a userDefaults.plist, defaults.plist or similiar. If you find one of those, then you are in luck, as the keys is highly probable to be keys of the deaults values.
You can even try it out, by specifying the keys on the command line, before writing them to the defaults database, to see their effect during the run-time of an app. (The app should be closed before starting it up with the arguments.
Be sure not to save anything as you peruse one such property list file, asit may break your app if you manage to.