Add a file .iiimp to your home directory with the content:
iiimp.server=iiimp://lindenberg:9010
Run a Java application that opens a window, e.g., IMFTest in test/java/awt/im.
Pull down the window menu by clicking into the top left corner of the window. There should be a menu item "Select Input Method" (or a localized equivalent), indicating that the IIIMP extension has been loaded and properly initialized. In 1.2fcs-G, the menu item is there. In 1.2fcs-I, it isn't.